That's where the law can get a little complicated and subjective. Basically, are you presenting it as a class or as daycare? Are the kids doing martial arts (or dance, gymnastics, whatever) the whole time, or are they hanging out and playing with toys and stuff? Are they there for an hour? Two? Three? If you call the parents will they immediately come get their kid, are you expected to handle injuries, medical stuff, upset kids, etc?
How do they pick the school-aged kids up from school without a shuttle bus?