Pictured: KevCon, one of our house cons in Kanagawa, Japan
A game that I would suggest is Shockforce. The rules are completely generic but it does come with a post-apocalyptic setting that you can use if you want.
The trooper design system is the first gem here. You
can take any mini you have and give it whatever stats you want,
weapons, armor, etc. and cost out the points and there ya go. It does not tie you down to any universe or line of figures.
The second highlight is the rules are easy, intuitive, and use opposed dice rolls so both players are involved in combat. Each player takes a turn activating-moving-shooting, a squad or character so again there is more interaction. I can yammer on about this game as it is one of my favorites. I can send you more info later. I think the kids would get into it as well as it is so easy but rewards sensible tactics and gives players some tough decisions about what to do with their troops.
Get on the Shockforce Group and get the Space Marines and Orks characteristics and weapons stats to give the rules a test run.
Set up 5 Marines and 10 Orks and give it a whirl.