How tall is your 12 year old? I’d say the kona is too small, fitted my ( tall ) 8 year old, but not much room to grow, looked about right.
We own a Norco b line 24, comes up a little bigger than the kona stinky 24, nice bike, still fits my 11 year old.
We changed a lot of parts on it as it was very heavy, now still weighs 34 lbs. Needed some air forks too as my son weighs 4.5 st. He does love it though…
I think commencal do a kids dh bike, I saw it in the alps last summer and it comes up more like the norco.
I did see local kids there ride small 26″ frames with 24″ wheels and short cranks, presume that when they grow they will just change the wheels to 26″. Maybe that would work?
I do think all kids 24″ dh bikes are for around 9-12 year olds…..