This is not sufficient grounds for the camp to expel the 8 year old girl. There is something a bit "off" about this 8 year old, and she totally deserves to be more closely watched by the staff at the camp. To call what she did "child sexual abuse" is stretching that category somewhat further than I would. See it as a learning opportunity for your boys, which it sounds like you did do. Sometimes it takes participating in an inappropriate scene for kids your boys' ages to grasp that it really is inappropriate. How wonderful that they opened up to you about it that very day. That's what you want them always to do. Catastrophizing the event might inhibit them from being that open in the future.

It would be okay, IMO, to follow up with occasionally asking the boys, "So how's that girl at camp acting?" The girl most likely engages in other behaviors that are not totally wholesome. And she won't be the last such youngster that your boys will encounter. I would explain to them that some kids come from homes where they don't learn how to act right and that it's too bad for them that they don't know better. I'ld explain that learning how to act right is just as important as learning numbers and how to talk and how to do all the things that kids have to learn. I wouldn't overly sexualize the message.

There's lots of good advice in the posts above.