My almost 5 year old is having the same issues with neighbours girls who are a bit older. I try to help her, but I decided to send her to a martial arts class where she can build her self esteem by being more confident with her own body and move up levels etc so she can see herself improve. Its based for kids with OT issues like sensory integration whatever, but they have different aged classes. If you call Kids OT in Frenchs Forest and ask for info about a martial arts class they can give you the number to phone.