I don't understand why parents allow their young children to watch movies, T.V., and Internet sites made for teens and adults. And what's with parents who buy their girls toys like Bratz dolls which look like tiny hookers? Then the same parents are shocked when they read these kinds of stories (children acting out sexually).
Of course even those parents who do monitor what their children watch will be exposed to other children who aren't monitored, but building a strong moral foundation at home will go a long way in combating the influence of those kids who expose them to things you don't want them exposed to.
Parents need to help develop their child's self esteem, especially girls. Personally I think a good way to do this this is at a young age expose children to experiences; rather than spending money on toys (which are mostly crap anyway) buy them some kind of individualized lessons (1 on 1) be it in sports, music, dance, acting classes, or whatever which will help them build self confidence in themselves thus leading to better self esteem and will more likely influence others rather than becoming the one whose influenced.
Finally, I think it's important for children to see other places and cultures (travailing abroad particularly) so they know a whole other world is out there which they can look forward to, I think this can help alleviate depression (thus making them less impressionable by others) by knowing that a whole different world is out there if they happen to be in a situation they don't particularly like.