So I can look at 650B’s without worrying about the large wheels – lots more choice
Not entirely…
It probably depends what type of riding and their weight as well.
My 7yr old can “ride*” my medium 650B (*as in down a fire trail) but he wouldn’t stand a chance on a technical run
The Cube has really really long (426mm) chainstays (longer than my 27.5 trail and about the same as my 27.5 XC) so if they can manual the cube easily then you probably have more options as most adult bikes will be as easy or no harder… but that then depends which bike…. If you are buying an XS or S then it would be better for trails to buy one designed as a XS/S (i.e. shorter chainstays) rather than a medium that’s just a shorter top tube.
The other thing is if they need their weight back then obviously the 27.7 tyre will take more space (I find on the kids 24er he will be touching the wheel with his bum)
Overall, if you want to ride trails I’d say find a trail hire centre with a XS and do a half day’s hire… see how it works out and what if any specific problems he has