Great question Johnny
Start off with a simple plank but as opposed to holding it statically I would suggest some foot movements.
Here is what I mean... From plank position (from elbows if limited upper body strength, hands if possible) lift right foot and pull knee toward chest and tap right foot on floor before returning it to the original position. Repeat with left leg and alternate back and forth. Build toward doing this action for 60 seconds. You can add variety and function by tapping the foot out to the side or crossing under.
Once they can do this for 45 to 60 sec they are ready for suspension.
Now put their toes in the foot cradles but leave their knees on the ground for the moment. To begin with have them lift their knees off the ground so they are fully suspended for as long as possible and then place them back on the ground again. Do this in succession until they have a combined time in suspension of 60 sec.
Once they can stay suspended for 15 seconds without returning to knees you can add some low amplitude pendulum swings.
At this point you are well on your way and can start to have some fun with them. You will be amazed at the progress that you can make and how much fun your clients will have.
Please keep me in the loop as to how it goes :-)