As requested, here are some Space Hulk rules for kids. It takes out the more complicated elements (like command points and marine turning rules) makes it easier for marines to kill things and makes the games shorter. These aren't playtested so give suicide mission a go and see what happens. I hope this is in some way easier to explain to your son.
Remove turning rules and command points. Marines can turn for free but can't change facing in Genestealer turn unless attacked.
Shooting hits on a even number and misses on an odd number.
Sergeant gets two dice and can add one to his roll. (no parry)
Overwatch - CP tracker is set to six every genestealer turn. This allows the marines to make six overwatch shots every turn. The marines no longer have to go on to overwatch to do this.
Heavy Flamer - Same as usual
Assault Cannon - Fires three shots and kills on any even roll
Librarian Powers - 1 a turn
Mind control - control a genestealer to move 6 squares OR attack an adjacent friend
EXPLOSION - make a genestealer explode! Every genestealer next to him is killed on an even roll
Follow me on Twitter @Adamwhitepoet