I would recommend taking up a martial art for most kids as it really does boost their confidence and help with self discipline. The important thing to do is find a good class in a style that suits your child, be that Judo, Karate, ninjitsu, capoera (spelling may be wrong), aikidoor TKD.
Beware of instructors who want you to pay in advance by direct debit. Most classes should let you pay as you go. Also each person in the dojo should have a personal license which is their own insurance as well as somewhere to record their gradings and any outside courses they attend.