At 6' tall, I am guessing there doesn't exist a seat post long enough to accomodate your legs on a kid's bike. Heck I am 5'2" with a 26.5" inseam and on my 37 cm MTB frame I have my 350mm seat post PAST the minimum insertion mark and it is still too short. The longest seat post I have ever seen is a 450mm and it only came in one diameter.