Gymnastics is a great option. I do not, neccesarily, refer to formal gymnastics training but some gymnastics movements supervised by a trained gymnastics instructor. They should find it good fun, too!
what does cns stand for exactly…i think i must be naive or not well read…
Central nervous system
I actually think that could be a good idea. I think it could help a person build strength in the same way that gymnast build strength. Their own body weight would be a breeze, and the coordination and rhythm picked up would go a long way in sprinting. Same goes for capoeira. It helps build core strength in a much more natural way without even going to the weight room.
Gymnastics can be good, but for all the good things it brings to the table, it also brings 2 bad habits with it. At least in my experience. I had a female 200/400, long/triple jumper who was a gymnast and when ever she started off down the runway guess what she looked like? A female gymnast coming down the runway to do the vault, or a tumbling exercise is what she looked like. Her arms were just as low, and she looked just as stiff.
The strength and flexibilty she gained from gymnastics is unmatched. She has one of the best looking starts naturally that i’ve ever seen. I don’t know if that had to do with gymnastics, but I didn’t have to do much work. She has a lot of coordination for not being able to dance. She picked up every drill quicker than everyone else, and her form improved drastically but much faster than most. Again I don’t know if that was because of gymnastics or me being a genius 
I think swimming would be the worst for a sprinter…Sorry Clemson