I suggest a tiered program:
1) Taekwando :Merely for the advanced kicking, flexibility, internal toughening, and endurance levels. Poor hand tactics however... and purely based in sport. Better kicking ability than
typical Karate classes. Stay in this art for only maybe 2yrs tops.
2) Akido :My friends kids took it, young boy & younger girl, and they love it.
Has plenty of things to keep busy with: Sensitivity training, locks, throws, weapons, and more.
Much more advanced than the typical watered down sport arts... However, not every kid
has the patience/focus level at certain ages for this.
Unlike Karate (and or some other "Hard-Arts") ... where the larger person has a greater advantage... Akido teaches how to flow around brute force, and use it against that person.
3) Wing Chun :In their teens... for the very advanced stuff. Similar to Akido with its use of
flowing.. but is a much more aggressive and advanced art. Techniques are very precise,
and require a lot more dedication and focus to master them. Very combat effective in
only a few years time.
Wushu might also be a fun consideration for the younger set. However, as cool as Wushu
can be... it no longer is based on real martial arts. Its more or less advanced acrobatics now.
I reccomend not staying too many years in the basic arts. They basically just repeat the
same things over and over again. While its good to get a solid foundation, too much is
simply redundant, and you could be spending efforts on learning more techniques and skill
sets in other arts instead.