My 9-year-old has been riding MUni on the 20" Pashley (see link). It’s been a great ride for him, but the “factory” tire is a bit on the lean side…really more on-road than off-road.
I’ve got the Maxxis Max Daddy 20" x 2.25" on my long jump/freestyle/some trials uni and it’s working really well. I took it out on Mt. Cootha for a 20" muni ride on sunday and it was great. No negative comments.