The Westermarck Effect I think works both ways. An adult who spends much time with an infant or toddler will be less likely to find them sexually/romantically attractive as they age barring extraneous circumstances. Then again this ignores nepiophiles (or nepiosexuals?). As for the kids themselves, it seems that this Effect really takes hold once they get into mid-puberty. Since kids generally seem pansexual or act in such a way. Social situations have really messed with research in human sexuality, especially child sexuality, it's hard to say.