As a young gamer (20 - began miniature wargaming at (bloody hell...)

I guess I can provide some ideas.
My father used to do scale modelling. My first memory ever was him cutting his finger open doing a Revell Dr.1. I was about 2 and a half at that time. So, yeah, never too young! I was doing models under his supervision from when I was about that age. I've basically taken up the mantel at this point.
For the rest, I'm not so good with advice. Warhammer - specifically the Lord of the Rings stuff that came out in deagostini magazines, was all the rage at my primary school. A warhammer group (where basically all anyone did was create "terrain" out of polystyrene foam) ran for a while at lunch. Being only an hour, you couldn't "run" (remember, these are primary school kids (i.e. 9-11 or so) a game and any attempts usually ended when the bell went before a turn had been played! So, first lesson, make sure you've got time! Lunch time won't cut it.
The advice I've seen from elsewhere is to go for skirmish games and (especially for kids) make sure there's lots of character. The reason everyone went for Lord of the Rings was that the movies were coming (or had just come out) then and that was in everyone's mind. So something topical would do too. (I can see X-wing doing well, but I've never been into Star Wars or the game itself so can't comment.)
Perhaps painting (with figures you can afford to lose, remember, these are kids, I cringe every time I open my old Warhammer figures) as well, keep it cheap and cheerful, don't expect a mona lisa or anything. But if you've painted something you become invested in it.
Our area was pretty wealthy, these are kids with pretty rich parents. (To keep up I had to beg, borrow, busk and anything in between to keep up in the arms race - hint, I lost)
And don't be disheartened when they lose interest.

That's about all the two cents I can give.