I recently began babysitting my friend's three year old son.  I'm happy to help her out but I am finding that her child is very aggressive and is constantly attacking my 2 year old.  When I mentioned this to my friend she said that he's just being a normal 3 year old.  I find it hard to discipline him because he is not my son but I'm having a very hard time.  My daughter is scared to be around him.  Is this really normal 3 year old behaviour?  Should I tell my friend I don't want to babysit anymore?