I was reluctant to quote Lewis because there is more context to his ideas than can be conveyed in that quote. The negative behaviors you describe aren't typical of just children: they are typical of all of fallen, broken humanity. It's just that children are more honest in being direct about those negative behaviors. :)

Since we associate these negative behaviors with "childishness," we risk throwing out the good with the bad: we lose imagination, wonder, honesty, playfulness, etc.

I realize that you aren't suggesting the author has those negative behaviors. I'm only trying to provide a greater context to Lewis' quote. Perhaps I should have just stated this argument on my own; I apologize for any confusion.