Little Johnny Dorset is a wonderful character. The kid cracks me up every time that I read the story.
One word that I would use to describe the boy is "gregarious." It means sociable and fond of company. That's definitely true of Johnny. He's so fond of company, any company, that he thinks being kidnapped for ransom is a super great time.
Johnny is also energetic. Sam and Bill struggle to keep up with all of the games that Johnny wants to play. Johnny is great at making up stuff to play, which brings me to the third and fourth words that I would use to describe Johnny. He is creative and imaginative. The games that he plays with Bill are not board games that need pieces. Johnny makes up the adventures on the spot and uses the items that he has on hand. I used to think that Johnny was the most ill behaved and annoying kid in the world. Then I started having kids of my own, and I realized that Johnny is pretty much like most 7 year old kids.