Im sorry but that is totally unacceptable - she has totally had someone do or show her how to do this. What kids know how to give oral sex - that is just crazy.
I would keep my kids way from her, i understand that the father is your boyfriend, but your children’s safety and innocence comes first.
Just reading what you have said about the situation has gotten my blood to boil.... this little girl needs some serious therapy and you need to find out were she has learnt this from. And you also need to question your Boyfriends role in all of this. He is either the culprit or the enabler, i am shocked that he is so relaxed about this, it just makes me wonder what is really going on here.
Please put your children first, and dont allow for any situation were this can happen again, I would never leave the kids along again....
I am NOT a child expert so what my opinions are are just that....opinions. I can only go by my own parenting experience and what I have learned through my friends kids and their experiences, worries etc (I have a 7.5 yr old boy, and 9 yr old girl). I think children go through a time were they are curious about themselves and other children before they realize this is inappropriate to show their private parts in public and socially unacceptable. Both boys and girls play with their privates to some degree. My girlfriend's son would do it all the time and it would drive her crazy (he was 6 around that time). I would tell my kids that if they are curious about their bodies to keep that exploration in privacy and in their bedrooms. And that no-one was allowed to, or had any right to touch them. AND that they were not allowed to touch other kids privates. Their body was theirs, and private. My boy would occasionally put his hands down his pants absentmindedly when he was younger and I would remind him to do that in his room & never around others. So this is what I consider normal for children.
As far as this 9 yr old girl...she is older and has been told not to masturbate around others. Why is the father not concerned? And wondering why his daughter is acting like this with other children? She seems to not have any privacy boundaries and acting out in a highly sexual way. My gut feeling would be she has been mistreated in some way. I could not imagine a 9 yr old girl engaging in oral sex with other children if she had not been exposed to it from somewhere. All I know is that if I was in this situation I would be uneasy, and would talk to my doctor about the situation, get some guidance. And would not trust younger children to be alone with her till you get some more answers.