If you really want the child to flush, you can always have him/her eat lightly (no fat or protein at all) the day of the flush, and don't do epsoms. You don't need epsoms for a successful flush anyway. (See Class Coke flush)
Anyway...have your child take 1/4 cup of oil and use ruby red grapefruits for the juice portion (they are much sweeter) before bedtime. Have your child stop eating at 3 or so, and eat very little...fruits, bread, whole grains.....have your child sip water until bedtime, then take the juice/oil mixture at bedtime. Buy a bottle of magnesium citrate from the store, and have your child drink half of it upon waking.