Re:
The first full one mountain bike I made for the Retro Grom was a Kona Kula which was labelled 14 but was 12 inches bb to tt - very light and decent frame. I spent a lot of time (and money) on getting it down to below 9kg, and with short cranks. And with suspension that actually worked for a very light child - I couldn’t get any air forks at 24 inches but found that old Judy springs worked in Spinner forks, so it was possible to get it tuned right down for light weight. But it was SO light and strong, and it got my son into full on mountain biking in a big way.
The next bike was an Orange P7, which usefully had a rear disc tab. This was the XS, and although it measured at 12.5 bb to tt was quite a bot longer in the tt than the Kula. I ran this with air race SIDS, and at first 24 inch wheels and then 26 inch wheels.
Then a small COTIC.