Another consideration is whether you want them to learn the whole clutch & shift routine while they are learning to ride. The XR70 is an auto-clutch (I think), and the XR80 is a full manual clutch.
At 8 YO, I would lean toward an auto-clutch, since you're likely to be upsizing soon anyway. Kinda depends on how much latent gearhead tendencies you've seen in them.:)
BTW, there's probably no reason to go new on the first one, since any used XR70 (or similar bike) has not likely been used that hard. Only problem is finding them on the market, but you're coming into the buyer's season.