My boys are 9 & 11,
11 year old is on a 14″ Scandal with Fox forks, running an XT 2×9 set up – kids don’t need a 42 tooth ring, all pretty much 2nd hand stuff, I’m impressed it’s a light weight build. His wee brother is on a BeOne Karma 15.5 Hardtail, dual air Rebas, again XT 2×9, probably not as light as the Scandal but shorter in the top tube so more suitable for him.
They both used to ride my wife’s Gary Fisher Tassajara, that’s when I realised that they could fit on and handle small 26″ bikes. They didn’t want her bike as it only has V’brakes, TBH, they’re better off with disks the speed they seem capable of getting up to.