Now - a lot of coaches might get mad at me for this one, but it works for me...
First, make sure they have good backward rolls. Without using their hands too much to get over. In fact, back extension rolls, using the knees as a driving force for rotation is ideal.
Now, to get over that fear -
I take a porta pit (or 3-5 stacked 8 inchers) and pop a wedge mat on top, angling inward to the mats (so the tall side is at the edge).
I have them first work on jumping up and back to their backs, and they will find that if they do it right, they will naturally start doing backrolls off. When they are comfortable with this feeling, hand spotting them and correcting the jump is a breeze!
A lot of coaches will either:
a) suggest you hand spot with the mats still there, without the wedge, so they feel the upward rise instead of a drive backwards.
b) disregard this entirely because it teaches a really bad habit of dropping the shoulders.
for those B people, it certainly helps those with fear to get over it. In my experience, I took a class of 8-12 year olds who wouldnt do a hand spotted or belt spotted back walking in the gym to doing them with hand spots into an 8 incher in about an hour! (of course, these were kids with some experience before working with me too, but not with back flips).
It works for me. If you try it and it works, GREAT!
If it doesnt work - well, at least you tried something.
My only caution is watch for their necks. They need to keep their chin in as they roll backwards, or they will start to complain about sore necks.
Also, put an 8" mat behind the setup in case anyone rolls fast and falls backwards. Comfy landing zones makes for happy bodies.