Josha & LE: Thanks but rules aren't the problem, it's the plot. I should have specified. I've run a game with one of these kids when he was 7 and he got what was going on. (And we were playing 3.5, which I think is a tad harder.)
Warlawk: Ooh. That sounds great! And I agree, 9 year olds aren't stupid. But I just thought of something. Since one of the kids has a starter kit of some sort (Im still not sure exactly what that is) so maybe that has some pre-set Adventure... (But he's probably read it, so it may be best not to use the pre-set. Or just alter it.)