hmmmm well, welcome. and i dont think so really..... a 6 year old is very small so i doubt he'll be able to literally get his leg around an atw or lowerbody trick, regardless of the material or size of the ball. of course there are exceptions, like this kid who looks about 8
depending in the level your kid is already;
if he cant juggle up to ten, then id say yes sponge would help. obviously its best to learn with a leather ball, but to get th basics sponge would give him more time with the ball in the air, then he could progress.
if he can juggle fairly well like the kid in the video, then for learning tricks i suggest a leather ball
with a sponge ball doing an atw it can go to high and get caught on the back of your leg. it happens to me, and im around 5"7. so the heavier the ball the less likely that is to happen.
if youre kid is interested in learning stalls (balancing the ball on head, neck, shoulder, chest, knee, foot etc) then i think a leather ball as well. with a sponge ball it has too much bounce to cushion anything very well, and can easily get blown off.
good luck to him
thanks for stopping by. if you need anything else just ask, ok?