Knuckles
Lives under a bridge
This is the way to go. Decent 24" wheels are even harder to find than the frames. Hell, even 24" rims are like finding a Charizard.Doubt you'll find one. A small or XS frame will do the trick. You will have trouble finding 24" wheels with disc hubs as well. Why not put the 8yo on the Riprock and the 10yo onto 26" wheels?
The 26 will give more longevity, with a 24er being a 12-18 month proposition at best. And if you use decent parts, you can move them up to the next frame size, reducing the financial burden of the little bastards growing like weeds.
#knuckleswasadrunkenchunt