Hokay. I HAVE ADHD as an adult; have had it since first diagnosis...at 4 years old. Teachers sent me to a shrink in Hawaii cuz' I wouldn't sit still: He called it ADD, put me on Ritalin. Over the years, through grade school, they eventually tried everything but the kitchen sink on me (med, dietary changers, behavior mod., etc.).
That being said, ADHD is a waaaay overused Dx, prompted by lazy & slow teachers who don't want to deal with the simple fact that kids are not designed to sit still for long periods of time. I deeply believe that MOST ADD is just kids being kids, so we medicate them into a manageable norm.
Now, to karate...
Training in Hawaii is what saved my ****. At the local Japanese Cultural Arts center, highly structured, militaristic training in aikijujutsu & kajukenbo provided structure. ADD minds can/do go to a lot of places at once, instead of only one or two places. Paying attention to weight distributions, hand and foot positioning, changes in tension, etc., gave me a thing to think on. In less militaristic atmospheres, though, no thrive.
Moved here in the early '70's, and started American Kenpo at the Garden Grove school with Bob Perry. Was a disruptive force in the kids class, but actually did alright in the adults classes (quicker pace -- more to hold the attention). Of course, he still couldn't stand having a hyperactive rugrat terrorizing his school, but it kept me busy and happy. There was at least one environment where my ADD did not prevent me from participating.
I can't say I'm any good at teaching ADHD kids. For kenpo, however, I would ask Bob White. He has what I consider to be the best kids classes available. He and his instructors have great patience, and maintain a positive atmosphere. For a kid with ADHD, this may be the only place where positive reinforcement or nurturing input may be found. Schools don't know what to do with them; families are often stressed to their limits by them. And if the only message a kid gets is that they're a problem, they will be.
Good Luck,
Dave