I studied Judo when I was younger ie 10-12
Its a more low key styled discipline (at that age anyway) heaps of throws and I remember my sensei was strickt didnt tolerate lateness or talking when he was, ie)pressups, situps, laps for punsihment. Id say look at judo
Also done jiu jitsu/jujitsu for 3 years which was really good, locks, chokes, grappling quite a bit more physical and competition orientated. Doing Ninjitsu now which is the best one yet (most likely the one i will carry on with for a long time) Uses a variety of weapons, swords, staffs, nunchuks, ropes, chains ....
awesome quite a bit more brutal for the youg ones, wouldnt let my son do it until hes 16+
The training is full on and we cover so much, disarming armed attackers while your unarmed/armed. Really good.
Ive heard good things about Aikido to was tempted to take it up a while ago but wne twith jujitsu instead. in our area alot of kids will take up karate as it is so wide spread and common. Judo is one i would recommend