Games Workshop recently released a kid-friendly (or at least as kid-friendly as they can be) version of Warhammer 40K called "Battle for Vedros." The box set includes a scaled back version of the Black Reach set from a few generations go: Space Marines and Orks. Also included in an introductory rulebook with a very simplified ruleset of 40K intended to be used as an introduction to the game. There's no army lists--you play with the miniatures you have, although they cleverly have "reinforcements" that you can add on with each side having an equal number of reinforcements (which I guess work out points-wise with each other).
The rules are mostly compatible with 40K, but are extremely unified. For example, all Space Marines hit on a 3+, all Orks on a 5+. Instead of templates, the missile launcher in the tactical squad just gets three to-hit rolls, the flamer automatically hits three times. Almost everything is a 4+ to wound. Where it begins to diverge from the regular rules is the Space Marine Dreadnought, which instead of having vehicles rules it just has the special rule that it can only be damaged by the Ork Warboss' power klaw or the gyrocopter's rokkits. It's also completely nerfed offensively, removing its power fist and multi-melta in favor of just giving it extra attacks (two shots, four close combat). There's no attack bonuses for charging, no leadership rules or breaking--you just lock up until one side or the other is wiped out.
There are no rules for cover, special deployment, or specific scenarios. It's a straight up fight until one side is wiped out (because Victory Points would require real units or points, which the game doesn't really have).
For $50 you get a pretty good deal in terms of miniatures. You get one SM captain, terminator sergeant, 7 tactical marines, and a dread. On the ork side is a warboss, 12 ork boyz, 5 nobs, and the copter. The odd one out of the group is the terminator sergeant; any terminator unit you buy will have its own sergeant.
But in any case, I bought the game (despite having all the minis already from an earlier Black Reach set) because my son wanted to start playing the game and it seemed a cheap price point entry since we don't own the most recent rulebook. I figure if he paints up the orks and plays enough games I'll consider investing the rulebook and go from there. If he drops it, I've saved quite a bit of money.