I don’t have kids myself, but I have been training lots of Circus/unicycling groups for kids. What really is the most important, is the selfmotivated will to practice.
My brother learned to ride at 5yo, on a 16 inch, but he really wanted it, so he practiced a lot. I learned to ride at 7yo ( as soon as i would fit on a 20", and I just could not wait to grow enough to be able to start), needless to say, I was really motivated and practiced lots, all completely self initiated.
On the other side of the spectrum, whenever I give a unicycle workshop over multiple days (usually about 4 days, 5 hours each), there are usually one or two kids that are just there because their parents wanted them to, and they usually learn extremely slow. I usually manage to get most kids from non riding to at least riding across the gym (about 40m/120ft) on their own, exception are the ones that are not motivated, and some rare cases of kids that need a bit more practice on their balance/motoric abilities.
In my experience it is important to give kids enough variety and breaks in order to not overload them, the first step is always practicing to ride with something to hold on to on both sides, in between a wall and some garden benches for example. Then move on to only holding on to one side (only a wall), with assistance of someone (first on foot, then on a unicycle when they need less assistance) riding away from a secure starting point (some lines on the ground are a great thing to show progress) and mix it up between those in order to not become to boring.
But yeah, forcing a kid to learn/pushing it on them to much is usually pretty unsuccessful, and not fun for both the kid and you.
As for wheelsizes: I have never seen anyone learn on a 12", but lots of kids learn on a 16". I also usually advise parents not to have their kid switch from 16" to 20" as soon as they fit, and instead wait until they have grown a bit more, otherwise 20" is just proportionally a huge wheel, and 16 inch a much more manageable size. (A 6 yo on a 20inch often looks like an adult on a 36")
I agree. Exceptions proof the norm, but I’d say 90% of kids that age possess the necessary abilities and especially important: the necessary endurance to keep on practicing.