Welcome to the forum GivAgo you can never have too many unicycles. sounds like you are finding some good deals. I’ve taught my fair share of young children how to ride a unicycle and, when it comes to wheel size, I’m convinced that bigger is better. Build momentum and a 36" practically rides itself. Mine started at age 9 sitting on an unrideable 16" rocking in front of the TV. Never pedaled a full revolution until I purchased them a 20" Nimbus Club with 114mm cranks and, with a little practice, were riding 20 to 30 foot increments by age 10.
It wasn’t until I purchased a first run 36" Nimbus Nightfox (an absolute game changer for the vertically challenged) that they really “got it” and took off for miles. Dual hole Spirit cranks (on a steel disc hub) on the Nightfox. 127mm (as long as will work) for the kids and 110mm because Daddy rides in bicycle events and I want them to one day experience the joy of spinning small circles with just a flick of the ankle.
It was a full year after my oldest met the minimum inseam requirement of the Nightfox and was tall enough for my 26" Nimbus Oracle (137mm hole of 165mm/137mm dual hole Moment cranks). I switched the heavy knobby Duro tire back to the Maxxis Highroller (same as Kris specs on his KH26er) I learned to MUni on. She loves “her” 26er Oracle. Her PR is 4 miles of rail trail.
I don’t believe in waiting too long before going off-road. It’s really safer and tons more fun. Don’t let them talk you out of 165mm cranks on a 26". They come in handy when it’s time to push that big cushy Duro uphill off-road. Though, I agree a beginner under 6 foot is best on a 20" unicycle with reasonably short cranks (114mm is good for basketball and street hockey) to start. My kids learned backwards 
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