Imo, most MAG and (especially) WAG coaches don't use trampoline nearly enough. It's EXTREMELY useful for training aerial awareness and control.
Most important part of training trampoline: teach kids to be visual. They should see the ground at every opportunity. For double backs, they shouldn't just see it at the end; they should see the ground as they finish the first flip and start the second as well. This will make it more consistent and less scary, and on top of it they can get in a lot more reps on trampoline than they can on a bar within any given amount of time.