kids teaching my friends sister dirty things.?

[Dory V]

well.. theres my best friends little sisterwho is like 4. and theres another little kid down the road about two year older?.. and her and her even littler brother are teaching her really bad things. like cuss words, about sex, french kissing, showing her bad websites. and it's gone too far now. they forced her to take her clothes off. and they showed her their private parts. the boy even had her touch his "private". i tried telling her parents, but they think i'm lying. i don't know what to do now. she still plays up there... because my friends mom doesn't beleive us either.... so her mom still tells her to go play with them. help here? i'm really getting worried about my friends sister.