I think it's because children are growing and not full-grown like adults. It's like a tree: whack a full-grown tree with an axe and it will lose some bark, but its defining shape will still be the same. Whack a still-growing tree with an axe and not only will it be damaged, it will grow crooked as well.
The pains we endure when still in our growth fase determine the rest of our lives. Add to that that children are still children and guilty of nothing and you've got a pretty good reason to protect them, I guess.
