If you can find copies, the "Test of Arms" series (Test of Arms, Battlefield Europe, and Sands of War) formerly published by GDW might fit the bill as squad level games with minimal rules. Units are squads and individual vehicles for the first two. Sands of War uses essentially the same system, but ups the scale to platoon sized units. The rules are very simple (4 pages basic, 4 pages advanced, and two pages optional for Test of Arms), and each title includes a large number of scenarios. Avalanche Press's "Panzer Grenadier" series (up to 4 titles now) might also be worth a look. These operate at a unit = platoon level, play well, and come with a very large number of scenarios in each game. They may be a little too complex for a 12 year old though (ca. 14 pages of basic rules).

I'll also add my usual plug here for TacOps, a computer sim of modern tactical combat written by a retired marine major and published by battlefront.com (not presently available for sale because the new v.4 is supposed to be released soon). This is an absolutely wonderful game, easy to play but very hard to win some of the scenarios. It wouldn't really be suitable for playing _with_ your nephews (designed for solitaire or net-based play, not FtF), but might make a great gift.