IMO the solution: don't make 8 year olds do top and bottom hand knee drills. Lots of more important (and fun) stuff to be working on. (not trying to be a jerk, just a different way of looking at it).
Also not sure how useful a top/bottom hand drill would be - their hand / arm / wrist strength is so limited, I think they'd be better served in the long run by learning how to properly get behind and through a low pitch, without resorting to lunging or bending at the knees.