A BMX bike is not measured by the length of the seat tube, or height. Instead, they are measured by the length of the top tube. You ride a BMX in a standing position, and it is designed to take jumps, and curbs, and even loops if the climb is steep and fast enough. It is EXTREMELY rare that the saddle is used, except during certain moves that may require it.

An adult sized, 20" BMX bike can, with EXTREME ease, be confused with a kid's bike. It is DANGEROUS for any small child who would try to ride one, because the length of the frame from the pedals to the handlebar can cause the rider to lose control of the bike and fall. An adult BMX bike is either designed to jump, or do tricks. It is not a commuter bike, and DEFINITELY NOT A TOY.