My 9 year old is riding a specialized 13″ frame with all my old bits on, pretty light and 1*10 with a 30T up front. Cranks are 155, Thorn ones. Even got him to help build it.

I’m not sure all 10 year old’s are the same height…but with a slammed seat, short cranks and stem and narrow bars i think it’d fit just fine.

I’m about to move my ‘taller than most ten year olds’ 8 year old up to a 24er / 13″ frame because i have one lying in the shed. I don’t kids are that fussy about sizing tbh. Soft seat and grips and some skull stickers are where it’s at.

Last year i built 2 bikes up for my eldest pair.
1 of them being a medium spesh pitch for my then 9yo and 5’1″ daughter
The other a short scott voltage fr for my then 8yo 4’10” son

Both run 1×10 11-36 cassettes with 30t NW ring up front,152mm cranks and 40mm stems.
Neither of them struggle for reach.

Our 9 yr old went onto a Genesis Core 20, 14 inch frame 650b, a few months ago. Fits him well and he loves it. High front end but not an issue when riding (stem has a few spacers above and is inverted)

Surely depends on their height? I’m 4ft9 and 14″ bike is just small enough. Shorter cranks, not too big tyres give you a couple of cm. however riding is bike that is too big sucks. If you live anywhere near Sheffield I have one in my shed you could try.

I did this, but put on a pair a 24″ wheels for a couple of years first.

My 8yr old is on a xs 26er. (13″ iirc). Fits well. Running 150mm thorn cranks, 80mm fork, 30t ring.

A good guide is the islabike range. Compare to that for frame geometry, wheel size, kid height etc reference.

My 4′ six year old rides a 24 … Can’t see how a 5′ ten year old wouldn’t fit a 26 small (13″-14″)