a coach who just tried to explain things to them, like "Straighten you leg, point your toe" sort of things. She would do it, or she would think so and it wouldn't be. We had a young coach come in, one year, and she would physically fix their form, as they went trough the motions, slowly. She would fix their arms by their ears, physically manipulate their body, where it needed to be straight, point the toe for them etc... The gymnasts would then get the feel of what those positions felt like.
my piece of advice is get them more comfortable on high beams. My beam coach always had us run and skip down the beam and back a bunch of times before starting complex, as well as doing bunny hops, walking on releve, pivoting, and tight straight jumps to stick. Just to get us used to staying square and tall on beam and make it seem less scary.
Just collecting a few gems of advice.
The idea that sglemom put out for correcting form and positioniong is a good one. I do that with a slight change by putting them into the desired position and then trying to force them out of it. The effort they put into remaining in position against my resistance helps them to immediately understand with out question which muscles to use and how those muscles need to feel. It is that "feel" that makes it possible for them to know when they're properly in the position.
Gymnast695 summed it up well by advising the floor lines and low beams for initial skill development, and the high beams for basic posture, take-off, landing, and turn drills, Just working stretch jumps or bunny hops gives them a tremendous lesson in how to initiate and finish skils that take-off from and land on two feet.
You should make a big deal out of having them do the "safe and easy" stuff as well as it can possibly be done. Celebrate out loud if a kid makes it down the length of the beam doing bunny hops with-out so much as a single corrective twitch. That's the stuff that really count's....making a single pass of warm-up drill with-out a twitch will show all the kids that it's not so hard to do after all. I mean geez, how can you expect them to cozy up to inverted skills and split positions when they can barely walk down the length of the beam.

X 10 = lots more