but also praises him as a hard worker. George tells Slim that Lennie isn't cuckoo or crazy, just "dumb." Slim, who gives Lennie a puppy, is perceptive enough to realize that while Lennie might not be very bright, he's kind and well-meaning.

It's Lennie's behavior with the new puppy that convinces Slim that Lennie is childlike. Lennie is as excited as a little kid to get the puppy, and in his excitement, nearly rolls over on the puppy. When the puppy has to go to the barn, Lennie goes with it, so enthusiastic over his new acquisition that he will, George says, spend the whole night there with it. Seeing this behavior, Slim says

He’s jus’ like a kid, ain’t he?

George agrees, saying the only problem is that Lennie doesn't know his own strength.

You can find many quotes in Of Mice and Men that reveal Lennie's childlike nature. For instance, when Steinbeck allows us to listen to Lennie's thoughts, we realize they are simple and often revolve around not wanting to disappoint George or incite his anger. He treats George not as a friend but as a parent figure whose guidance he relies on. Like a child would with a parent, Lennie also often turns to George for comfort, asking him to repeat again and again the story of the farm they will buy so they can live off the fat of the land.

Lennie shows his childlike nature in his love of animals and his desire to touch and stroke pretty things, be it the fur of a puppy or Curley's wife's hair. Like a child, he can fail to have good judgment about not hurting smaller creatures, often not realizing his own strength. He also uses simple words, just as a child would, because he lacks a sophisticated vocabulary.

A quote that shows Lennie's childlikeness is as follows. In it, Lennie, who has just accidentally killed Curley's wife, confesses to George as if he were his parent, using the simplistic words "bad thing" to describe his behavior:

Lennie said, “George.”

“Yeah?”

“I done another bad thing.”

This is not the complex language of an adult, who would probably rationalize his behavior, express extreme guilt, or try to blame someone else. Lennie, in contrast, can't seem to quite understand what he has done beyond that it is "bad."

"Look, George. Look what I done." This quote clearly expresses Lennie's childlikeness, but it doesn't suggest much about his strength. It is a response that a child would make when he or she has done something to be proud of. Lennie, however, has done something wrong in this instance and would obviously not want George to know about it, as has been illustrated in various incidents in the novel: when he tried hiding the dead mouse from George at the beginning, when he tried to bring a puppy into the bunkhouse and hid it, when he killed the puppy, and when he killed Curley's wife. 

Lennie realizes that he is now in terrible trouble with George and it seems as if he cannot fathom the real depth of his actions. He only thinks about the fact that he will not be able to tend rabbits, and when he says:

"George gonna say I done a bad thing. He ain't gonna let me tend no rabbits."

This quote, combined with the images you have created, clearly suggests that Lennie has used his strength to have "done a bad thing"--the visuals are an obvious indication of how powerful he is to have killed both Curley's wife and the puppy. The second part of the quote alludes to a child's reaction when he or she realizes that certain privileges will be denied for having done something wrong. In this regard, Lennie's childlikeness is most aptly illustrated. 

Here is a section the 4th page into my book:

George looked at him sharply, "What'd you take outta that pocket?"

"Ain't a thing in my pocket," Lennie said cleverly.

"I know there ain't. You got it in your hand. What you got in your hand - hidin' it?"

"I ain't got nothin', George. Honest."

The text goes on to declare that Lennie indeed has something in his hand. It is a mouse and he just lied to George about it. This demonstrates childlikeness in that a child will lie to save himself from sure punishment. This also demonstrates childlikeness because Lennie tried to be "clever" and George, the parent in this situation, saw right through it.

Later on the next page, George is testing Lennie about how they are going to act when they go and talk to the boss. After consenting to say nothing, George encourages Lennie like he would a son:

Good boy. That's swell. You say that over two, three times so you sure won't forget it.

This is how children are. They need repetition and praise. George knows how to handle Lennie. He knows Lennie needs each of these for success and Lennie eats it up!

In the opening scene of the novella, Lennie's adolescent nature is depicted by his reaction to George throwing his dead mouse into the bushes. Like a child, Lennie reacts by crying and struggling to control his emotions. Steinbeck writes,

"He heard Lennie's whimpering cry and wheeled about...Lennie's lip quivered and tears started in his eyes" (5).

In chapter 3, Slim allows Lennie to have one of his puppies, and Lennie becomes attached to his new dog. Despite George's numerous warnings not to play with the fragile newborn creature, Lennie sneaks his puppy into the bunkhouse. George then spots Lennie holding the puppy and commands him to bring it back to the barn with its mother. Slim then comments on Lennie's childlike behavior by saying,

"Sure he's jes' like a kid. There ain't no more harm in him than a kid neither, except he's so strong. I bet he won't come in here to sleep tonight. He'd sleep right alongside that box in the barn" (Steinbeck, 21).

In chapter 5, Lennie accidentally kills his puppy because he is too rough with it. Lennie expresses his displeasure and resembles an upset child by saying,

"Why do you got to get killed? You ain't so little as mice. I didn't bounce you hard" (Steinbeck, 42).

Similar to a child, Lennie blames the puppy for its death and speaks to it after it dies. Lennie's lack of self-control and responsibility also mirror a child's personality.

Shortly after, Curley's wife enters the barn. Lennie is hesitant to speak to her. Lennie insists on following George's commands, which again reveals his childlike personality. Lennie's reasoning for not speaking to Curley's wife resembles a child's logic when he says,

"George says I ain't to have nothing to do with you—talk to you or nothing" (Steinbeck, 43).

Lennie then begins to discuss his fascination with petting soft things with Curley's wife. Before Curley's wife allows Lennie to stroke her hair, she references his immature nature by saying,

"You're nuts.... But you're a kinda nice fella. Jus' like a big baby. But a person can see kinda what you mean." (Steinbeck, 45)

Now, look- I'll give him the work tickets, but you ain't gonna say a word. You jus' stand there and don't say nothing. If he finds out what a crazy bastard you are, we won't get no job, but if he sees ya work before he hears ya talk, we're set. Ya got that?"  (Chapter 1). 

This quotation shows the reader that Lennie cannot navigate the world without the help of George.  George tells him what to do, and he tells him how to do it.  Without George running interference, Lennie could not even get a job to support himself.  His lack of intelligence would scare away prospective employers, so George has to speak for him in order to help him survive. 

"I forgot, George."

"Yeah, you forgot. You always forget, an' I got to talk you out of it." He sat down heavily on the bunk. "Now he's got his eye on us. Now we got to be careful and not make no slips. You keep your big flapper shut after this." He fell morosely silent (Chapter 2).

In this quotation, George is reacting to Lennie’s inability to remain silent while the boss is talking to them.  Because Lennie forgets things, he is a liability to George, who must make excuses for him and constantly watch out for him.  If Lennie was forced to leave George, he would not survive.  He would not know how to find food or work.  If he found work, he would not be able to maintain the job without George telling him what to do.

Through working with Lennie in the course of a day and in seeing how appreciative Lennie was when he received one of Slim's puppies, Slim recognizes that Lennie is not deliberately mean.  He understands that the relationship between George and Lennie is one where the former takes care of the latter.  Inside the bunkhouse, this becomes evident to Slim in an exchange between George and Lennie:

Lennie sat up quickly. “Give ‘um to me, George.”   George said, “You get right up an’ take this pup back to the nest...You take him back or I’ll tell Slim not to let you have him.”  Lennie held out his hands pleadingly. “Give ‘um to me, George. I’ll take ‘um back. I didn’t mean no harm, George. Honest I didn’t. I jus’ wanted to pet ‘um a little.” George handed the pup to him.... Lennie fairly scuttled out of the room. Slim had not moved. His calm eyes followed Lennie out the door. “Jesus,”he said. “He’s jus’ like a kid, ain’t he?”

This excerpt is important in detailing Slim's understanding of Lennie for a couple of reasons.  The first is that Lennie immediately reverts back to a child when George threatens to take the puppy away from him.  When Lennie pleads, "Give 'um to me," and says "I didn't mean no harm," it becomes clear to Slim that Lennie has an approach to life like a child and is not deliberately mean or cruel.  Lennie does not challenge George's authority, but rather capitulates to it as a child would.  Slim notices this.  At the same time, Lennie's submissiveness when confronted proves to Slim that Lennie is "like a kid."  Slim has seen men like Curley who use violence and power to achieve what they want.  Slim has seen Lennie work.  He knows the physical capacity he possesses.  Yet, Slim recognizes that Lennie does not use any of that with George and even when he does not get what he wants, he does not revert to his physicality. Rather, Lennie acquiesces to George.  It is through Slim's interaction with other men that he realizes Lennie's differences and understands that he is more of a child than anything else.