I know this sounds vague but the rule I adhere to is that the CNS must be recovered in terms of days and minutes recovery between reps.
Two, the workout should scaffold and be appropriate to ability. Trying to have a kid go from a w.o. of 20m accel=>20m fly to a 40m fly without developing acceleration out to 25m simultaneously can throw the body off.
My favorite workout are “flys” just because you can concentrate on various elements of true speed with adequate recovery w/o thinking about not throwing up.