A lot of it is to do with the whole "innocence of childhood" idea that came through a lot during the 19th & 20 Centuries. I'm not all that well-versed in childhood studies. We used to have a user flaired in the History of Childhood but it seems no more. Anyhoo I've asked/answered similar questions so you might find the following interesting:

• History of Paedophilia & the Paedophile Information Exchange

• On the culpability of children with reference to the "James Bulger" case