I don't think that all of them necessarily are. But I guess often times the story requires the child/ren to be functioning at a certain level... and as in real life, particular situations will many times extend a child further than they would have otherwise been, had a situation not arisen.
Some, yes. All? Not in my experience. Children are just like adults in that respect. Some blossom under challenge and duress, others cave. I just wish King was willing to acknowledge the latter once in a while. His clear respect for little kids causes him to write them through rose colored glasses. Certainly not a deal breaker; he's still the greatest popular author in history, as far as I'm concerned. I just find the way he sculpts his most junior characters to be one of his weaker qualities.