Innocence is much more about a grown-up sense of loss than a precious quality of childhood. At best, our ideas about innocence evoke a pining sort of regret; at worst, they're used to make nostalgia a form of tyranny.
After all, it's a child's job to grow up, and so he's active every day trying to lose that innocence that he sees as keeping him away from adult freedom and power. Our attempts to preserve that child's innocence just reinforce his sense of powerlessness. This only serves to exacerbate his little rebellions, his need to prove how grown up he is.