S.,
I do not want to alarm you even further, but I believe your God-given discernment has caused you to recognize warning signs that something is not right. Please continue to listen to the proddings of the Holy Spirit; I must commend you for not choosing to ignore or deny what you are seeing and know in your heart. Don't let others convice you everything is ok and normal when you know it is not. If this was such normal behavior for a child her age, then why are the other parents concerned too? Shouldn't they recognize that this just happens at this age? NO, they are concerned the same reason you are, because this is not ok.
I agree with Cathy. You need to speak to her about this and ask her where she learned this behavior. This is neither normal nor appropriate. I think it is important to find out what is happenning with her, but without asking "leading" questions that might pose a legal problem later if there were a need to prosecute. I.E.: Ask "honey, where did you learn this? or Did you see someone else doing this? or Has someonone done this to you?' NOT "Did you see something like this when you were with daddy?" or "Did daddy or one of his friends touch you?". I'd be very careful not to include anyone's name or suggest any person when asking your questions. Also, be careful how you respond to your daughter. Do everything you can not to lay any blame or shame on her or make her feel like she must betray someone she loves. Try to ask casually. Maybe you should take her to a trained counselor that has been taught to ask these questions without sounding intimidating or condemning and knows how to be get children to tell the truth without suggesting their own opinions of what might have happenned. I also think there must be something you can do to ammend custody arrangements; Sleep-overs with daddy's girlfriends are absolutely inappropriate and as her mother you should have a say as to what you want your child exposed to, especially when such exposure is detrimental. Please speak to an attorney and child psychologist. Also, I will be praying for you that God will give you discernment into the situation so that you may know what is going on with your precious little girl and how best to help her and protect her. Remember, none of these behaviors should be something that she feels guilt for. I pray that God would bring you and your daughter healing and peace and that the Holy Spirit will protect your daughter from any long-lasting damage to her heart. God Bless!