Learner's cycle for outside

Hey folks –

Trying to figure out what 20" unicycle to get for my rough-and-tough nephews (11 and 13), and my daughter (not-so-tough 9). It must be of excellent quality, but we have no indoor facilities so it must be suitable for outdoors. I learned/am learning on a 26" Pashley.

Should I go with the 20" Pashley? Or would the Miyata or Semcycle XL be better? How would the freestyle cycles perform on dirt or gravel or grass?

Were it a matter of learning in a gym, I think I know the issues involved - it’s just the outdoors part I’m concentrating on.