as the parent of four, when I rented children's boots, I would rent through the pile for the newer ones (duh!), but newer boots are less packed out and therefore warmer.
I am less concerned with most of the fit zones on a kids boot, but have found that as long as the ankle is tight the kids can ski just fine. 6-8 years old is the limit for this rule, though.
If your kids are stronger skiers, you'll have to buck up and purchase good fitting boots. Been there done that.