You did not fail as a parent the only time you fail as a parent with this is when you knew and did nothing to stop it. There is no way anyone could have known without catching them or her or a witness speaking up about the situation. I would definitely be having it out with the other parent and figuring out how this could have happened. I would also inform the other parent she will not be coming back over anymore and that they will have to make a choice either the person they are now married to or their child especially if this comes out to be true and there’s evidence. I would inform them that legal action has been taken and that you will be in touch and inform them of what’s going on. The children that did this to her being so young I would definitely be suspecting if they were being abused either by someone they know or one of their parents and checking to see if it’s a learned behavior that they have picked up on. With your brave little one now she just needs you to be there for her and assure her this isn’t her fault she did nothing wrong that it was the other people you are to blame that have did it to her. Counseling is going to be her biggest help make sure you go with her even if you can’t be in the room just wait in the waiting area just for support even if she’s uncomfortable with you being in the room you can still be there. Reassure her that she is in a safe place with safe people that no one is here to judge accuse her or demean her in any way shape or form on what has happened or accuse her of lying. Try and help her find ways to cope with what’s happening check on her even if it’s just going into her room at night and peeking in when she’s asleep just to be sure she’s there and okay. It may take time for her to open up fully to you and tell you exactly what’s going on but all you can do is be patient and don’t push it. she needs to know that you are not judging her you are not going to accuse her of lying or blame her for what has happened and that you’re going to protect her and they’re not going to be able to do that to her ever again and no one else will either. I would stop visitation until they get to the bottom of this and figure out what happened I would try to figure out if the other parents have known about this or suspected it. There is a chance they may not have known just to reassure her that you both love her and no one meant for this to happen she’s not a bad person some people just do bad things. She may blame herself for not speaking up sooner or whatever the case may be or blame herself for what happened thinking it’s her fault just let her know it is not and she did the right thing for finally coming forward that she has protected them people from being able to do this to others and to her ever again. This is going to sound really weird but she should learn forgiveness she doesn’t have to forgive them but to forgive herself. She may have some emotional issues just be patient it will be hard she may have nightmares May lash out with sadness or aggression she may lose her patients a lot and feel lost but don’t fight back just breathe and just remind yourself she’s been through a lot she’s not trying to be like this she is trying to cope with what has happened.