The proper solution is just tell the kid to take a breather without stating a reason (maybe tell them privately how to notice on their own when they are overworking themselves). The other kids won't notice or care.

(And if some kid is looking for an excuse not to do the exercises... why are they even there? And who cares? It's an extracurricular, the point is to get out of it what you put in. It's not boot camp.)

First, you are literally punishing kid for being overworked, while claiming they are slackers are teaching them to overwork themselves even more next time.

The harder the kids try, the more the kid will be punished and claimed to be lazy.

Second, you are lying to the kids, they have no way to actually figure out rules.