I would recommend that in order to get your moneys worth as a parent, wait until they are 10. If you go to a decent school they will not give your child a full black belt until the age of 16. So if your child earns a black belt before that they could be a junior black belt in a "holding pattern", that is re-learning the same stuff until they reach 16. Not to mention that most kids under 10 do not stick with anything for too long.