parenting varies from parent to parent and there is never a wrong way to parent in my opinion... I know this may sound stupid but it has helped with other parents... google how you can talk to your child about this situation so he can better understand it... i know it can be horrified for you, but he does need to know that it isnt right and could really mess with his life and the girls life.. i hope this helps you and good luck to you
What ahs the parents of the 5 year old to say about it, i think you are right to be concerned and definatly he has to know he cannot behave like this with a younger innocent child , i would certainly ground him there has to be some repercussions for him to see the seriousness of it..indeed it could be a isolated incident I would make sure he is not alone with younger children again ...you could ask a counsellor what they think ...