When can I use self defence, and to what degree if I'm being aulted?

I'm currently on holiday and there is a group of aboriginal kids their ages are from about 11-15 have been attacking us, now on 3 seperate occasions. They hurl bottles etc and hit us wtlith their scooters. Me and are mate who are 21 basically walked of because the main ault was being done by the girl abogines kids whilst the males sort of hide behind them for protection! I feel powerless againts them! Also now my little sister are up here and to tonight after I left them they were confronted by these s and i'm scared for there safety, mind you I have already called the cops and they have spoken to them, but it's like they don't care! What I wanna know is, is if I am confronted with then again which I am 100% sure will happen what should i do? And can I phyiscally defend myself even against these girls?