My 10y old upgraded from MTB (24'') to a 26'' Gravel today and he is absolutely loving it.

26'' Gravel bikes are hard to find in Germany but luckily I found a distributor (Meipel) just 30km from where I live. Today we visited them for testing and pickup.

It comes with Microshift and a mechanical Tektro that was a real pain in the butt to adjust but for the price of 800€ it is totally fine.