Although I fixated on having a cassette wheel for the 24" bikes I'm not certain that a kid who is properly fit to a 20" bike can actually bend an axle on a 20" bike, the lack of gear selection becomes the biggest issue but I found my kids haven't shifted anywhere near as much as they should and having more gears wouldn't matter much. Critical is paying attention to crank length, the C-dale quick razorrock mentions comes with a 110mm crank and that is a good length. Swapping out a shifter cheaply isn't a big deal, getting a better crank length is not cheap by comparison. The 20" giant I bought for my son, who wouldn't let go of wanting the color, came with 150mm cranks; with seat height so he could touch the ground his knees came too high and he looked awkward when standing to pedal. A drill and tap solved the issue but not everyone can do that.