A good progressive edge all the way through the wake, standing tall at the top of the wake. This much advice can be had from just about anyone who has moved past the frustrating point your boys are at. I think many who were self-taught have been there.
I used to be a big slalom skier, and the whole edge-through-the-wake concept was counter-intuitive to me, as it's a hard slalom edge that's critical to cutting straight through the wake on a slalom pass.
You may want to get a video, such as Detention or Higher Education. Seeing people show the fundamentals is certainly worth way more than words alone can convey.
http://www.wakeworld.com/VideoGuide/ Once they get some good fundamentals down, they'll easily land way out in the flats at 50' (even behind your old Correct Craft at 22mph). They'll quickly want to move to 65' or more.