Dave:
funny u should ask the question...
over the weekend, i let my little bro (he'll be 7 in December) assemble one of my kits, and he did a great job ! it was a Sherman M4 in 1/72 by Armourfast...
they're designed as quick builds for wargamers; the kit has all of 7 parts to put together (upper hull, lower hull, one piece track/wheel assemblies for each side, turret, mantlet cover and main gun) and it looks pretty good when it's done. the kit cost about $10 (US), and the best part is u get 2 of them in the one package ! so, if u don't have anything against small scale (i know with little kids size does matter), this would be a great way to go !
my little bro' has some fine motor issues , but he had no problem with this kit, so i'm sure it won't be too fiddly... just pick up a tube of non-toxic glue, and ur good to go !
Armorfast is suposedly doing a few more subjects soon, including an M-10, but this was the only kit i've seen so far... BTW, they're a UK manufacturer, as u can probably tell by the spelling of 'Armour" !
best of luck with ur search !
frosty