hmmmmm - that doesn't make a lot of sense... While there might possibly be clearance to put slightly larger wheels on, what exactly would that gain you? The frame would still be the same size - if the kid's legs got longer than the seat post would accommodate, then you'd also have to change the crank length - then that would start messing with the knee/saddle position.... that would mess with the stem length. I'm thinking just using bigger wheels doesn't really change anything about the way the bike fits - it just allows you to do a bunch of other changes - like longer cranks and still maintain ground clearance?
Also I think that might cause major toe overlap - my Conquest 24 (I'm seriously short... the 24 is a bit small for me, but no adult cross bike was small enough..... I've adapted it a bit with a longer stem and long rail saddle) anyway... My Conquest is the only bike I have that actually has some toe overlap issues.