My two both do Taekwondo, DS is 9 and started first 4yrs ago, he is preparing for his provisional black belt next grading (adult proper black belt). DD has just started and will work through her kids belts.
They both go twice a week, DS sometimes three times. It is extremely physical - they do sparring, self defence and as they move through their belts patterns, one steps etc.
It is great for discipline, listening and following instructions - if they don't they are sat out of part of the classes and can have their belts taken off them for a week.
It's a big commitment if they want to really progress through, it's not enough just to do the classes once they start patterns etc, they really have to practice at home too.
We love it