You can use a 20" wheel on the 1up USA trainer.

http://www.1upusa.com/bike_trainer.html

Quote Originally Posted by 1UP USA
Vertical height adjustment for any wheel size, even kids bikes. Bicycle can be lowered for level riding. Eliminates need for costly front wheel block. Other trainers require additional attachments at added cost to accommodate smaller wheels. Our trainer fits all wheel sizes from kid’s 16 inch to adult 29 inch. Train natural, train level.