20" 451 wheel vs 20"406 wheel.
The most common wheel size for kids bikes, bmx, freestyle, is the 20" 406 wheel. Tires and tubes are available in virtually every bicycle shop in the World for the 406.
451 wheels are used on some high end folding bikes, some recumbent bikes and high end BMX race bikes for kids under 80 pounds. I would wager in the United states less than 20% of the shops carry tires or tubes for this size. 451 wheels are very rare. Tire selection is pathetic compared to the 406 wheel which has thousands of different tires available.
I owned and managed a family, BMX race, Mountain bike Race shop for 30+ years.