Tell the kids that a computer is a machine that pretends to be other machines. Programmers make up the rules that tell the computer what to do.

If you give a computer one set of rules, it will pretend to be a typewriter (though most 8-year-olds have probably never seen an actual typewriter). If you give it a different set of rules, it will pretend to be a CD player. If you give it another set of rules, it will pretend to be a DVD player.

Then forget the computer and have them 'program' each other. Have them decide what they want someone to do, and let them make up a set of rules for how the person is supposed to act. As the moderator, you can tell them when things need to be broken down into smaller steps, and put limits on how much the kid 'running the program' can know.

Encourage them to be deliberately obtuse and literal about following the instructions, so the kids writing the rules have to get more and more specific as they 'debug' their program.

That should be accessible for kids of that age range, and it turns some basic programming skills into the kind of competetive game kids that age enjoy.