Tougher question than it sounds at first.
First, Orbiter isn't easy - it's a real physics simulation. It's not beyond most people, but there is a learning curve. Do you (or the kids) have experience with other flight simulators?
You kind of have to tailor it to the kid. Kids seem to do best when the level of challenge is right - too easy, they get bored, too hard, they get frustrated. I suggest a series of tasks. Some will complete the early tasks easily, some will not - depends on the level of skill (have they used other flight sims?).
Consider spending some time with "Unlimited Fuel" enabled, just flying the Delta Glider from Habana to KSC or something.
A really good thing is to get "Go Play In Space" - it's the best comprehensive tutorial I've seen, and most of us started there. You can find it, and plenty of other tutorials, in the forum's tutorial section.
I'd use the stock DG at first - it's futuristic enough to be cool, and quite forgiving of the mistakes that are inevitable.
The first set of tasks would have to do with standard aerodynamic flight. The ability to take off, circle, and land again. Landing is particularly tricky, so different kids will learn that at their own speed. Such a mission could be a "Habana - Cape Canaveral" flight.
Second, I'd focus on orbital mechanics. Start in orbit, and and have to make a plane change, sync, transfer, and rendezvous with a target. Focus on the methods of the hohman transfer (changing orbital altitude), and plane alignment. Perhaps a MIR - ISS transfer.
Third, introduce launching to orbit. This uses skills used in both the first two "missions", and further re-establishes the differences between atmospheric and orbital flight. A "mission" would be Cape Canaveral to the ISS.
Then I would look into more advanced, "cooler" vessels like the XR series and the DGIV. These vessels have some damage simulation - so they can burn up during re-entry, etc - and some additional attitude control and autopilots that make re-entry much more realistic than can be achieved in a stock Delta Glider.
Then, if the kids (or you) are still interested, look into add-on navigation tools. There is a version of TransX that comes with Orbiter - but it is quite dated. There is a much newer (and better) version around here somewhere - probably at OrbitHanger. (See the "external links")
LTMFD is great for lunar transfers - and back. IMFD is the "Swiss Army Knife" of nav tools.
Whenever you have a specific question, there are many people who will be glad to help. Try the "forum search" feature first, and of course, Read The Friendly Manual, but when you are stumped, we are here.
This is not beyond the ability of a kid, but patience and some learning will be required.