Little kids that try to steal stuff

A girl tried to steal my phone yesterday night at Marble Arch. I was in McDonald’s. She came over to my table, draped a pamphlet for a watch or something over my phone, and said something about “50 pence” a lot of times. My spidey senses tingled. I said no, and then she started to leave, taking with her my phone. I grabbed her little wrist (and it really was little) and she let it go, and then ran off. What are you supposed to do about little kids that try to steal stuff?