A cutaway is a cool idea, but having taught Industrial Technology in Middle School for 5 years I would say that what you want to do is more suitable for the 15-17 year old age range. If you want to do a carburetor project, use a simple carb like a tecumseh lawnmower carb:
and just have him take it apart and then you both can epoxy the pieces to a board and he can make up labels for what each piece does. If you want to make a killer app for this, come up with a small clear plastic box and figure out how to put an old perfume atomizer that the two of you find at a goodwill store in it with the squeeze bulb on the outside of the box and put colored water in it, so people can squeeze the bulb and see the water magically rise in the tube.
Even what I just suggested would be a lot of work for a 12 year old, but doable and he will learn a bit more himself this way too.