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?
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 .?