I don't use sabots much anymore but I use to have a few sabot/bullet combos where the jag would get between the bullet and the sabot and I was afraid I was pulling it a bit off the charge when removing the range rod. Solution for me was either to use a shorter sabot or a different jag.
As far as the kids go I think it just depends on the individuals. Some 13 to 15 year olds are plenty mature enough to muzzleload on their own, I also know some people in their 30's who aren't ready.
Working with them you should get a good idea how much rope to give them.
Just my thoughts.
Art