There's a tricycle in someone's yard around here with a triple tree on it, with real cups and bearings, the front wheel is about 12".
There are LOTS of 20" bikes with triple trees, most are obscure, but they are around.
The small OCC Chopper has a 16" fat rear tire and 18" front skinny on a triple tree, but the fork is long enough to fit a 26" tire.