Interestingly, I looked at Universal Cycle's web page of freewheels to see what sizes are available, but what they list as 8-9-10 speed "freewheels" look like cassettes to me:

https://www.universalcycles.com/shop...?category=1665

Anyway, freewheels at least go down to 13t, still not enough for a 20 inch wheel.