There are multiple times where the monster shows behaviors typical of a child in Mary Shelley's novel Frankenstein. When the monster takes over the narrative (in a sense, given the story is Walton's telling of Victor's retelling of the monster's story), the monster tells readers about "the original era of [his] being."
In the very beginning, the monster was unaware of his senses, of light and dark, and of many of the elements which surrounded him. Like a child, the monster was required to learn about his surroundings in order to understand how to function within them.
Like a child, the monster is frightened when he discovers he is alone. This fear was innate (much like fears in young children).
It was dark when I awoke; I felt cold also, and half frightened, as it were instinctively, finding myself so desolate.
Not only that, the monster lacks knowledge about certain things--like fire. The monster, unaware of fire's ability to cause pain, thrusts his hand into the flames and is burned.
In my joy I thrust my hand into the live embers, but quickly drew it out again with a cry of pain.
Like a child, the monster's lack of knowledge regarding fire caused him to be burnt.
Overall, the monster's lack of knowledge about the world around him (when he first comes to life) proves him to be very innocent and childlike. The fact that the light of the sun and the dark of the night frighten him show him to be needy (like a child who needs a parent).