I think sleep overs for young children are not a good idea ..many time things like this happen, the age group is also too big even if the child has a capacity of 6 ..Maybe wait till he is a lot older.It may be a good idea to ask the doctor what he thinks regarding any affects ..is your child speaking about it to you .?
Gosh, that is terrible. It is odd that someone saw this repeatedly and didn't stop it or seperate them. Who was watching the kids?