Looking at the trek or salsa they come equipped with 140mm which to me is too long for a 20" bike, you can watch the kids bobbing when they stand to pedal. The specialized has 127mm which is more appropriate though it goes to 7sp. This is the problem with kid's bikes is the balancing act you have to do for cheaper bikes. I will say with the chainring size that a 20" bike comes with having bigger then a 30t made no difference to my kids when it came to climbing. We don't have major climbs around here which might be part of the reason but for rolling XC they've got no trouble and usually when they walk a hill I discover they haven't actually used their easiest gear or two.