My 10 y.o. son has been transitioning from a Marin Hidden Canyon with 20" wheels to a 24" Specialized Hotrock which he is riding until the 24" Cleary bike is available in September. He mentioned that he had a tougher time climbing and cornering on the Hotrock than he did on the Marin. It seems to me that the Hotrock is a little slow to turn into corners. I remember when we got the Marin that it was a lot shorter than the singlespeed 20" Hotrock that he rode for a while. He didn't like that at first since the longer Hotrock felt more stable, but quickly like the shorter Marin better since it was faster & more intuitive feeling on singletrack.
I checked out the specs for several bikes and indeed the wheelbase on the 20" Hotrock is about 1.8 inches longer than that of a 20" Marin kids bike and almost 3" longer than that of a 20" Cleary bike. Out of curiosity, I just did some research on kids' 20" & 24" bike geometry. Specialized does indeed have very long wheelbase frames. It seems like the "enthusiast" brands offer frames with especially short wheelbases which may handle better under kids with some trail riding experience.