In mice and men what did Lennie do that was childish?

Answer 1

He copies George, and likes soft things.

He also, as I quote, say things something along the lines of, "Look George! Look what I did!" - When making ripples on the water of the pond in the beginning of the novel.

Answer 2

Lennie often engaged in childlike behavior such as playing with small animals, being overly dependent on George for guidance, and not fully understanding the consequences of his actions. His simple-mindedness and innocence contributed to his childlike nature.