When it comes to jumps its about both, strength and flexibility. I've seen flyers that can bend their legs behind their head with ease and then barely get off the ground in a toe touch. You first need to concentrate on strength in the hip flexor area, core (lower ab specifically), and leg strength.
Hip flexor exercises (sit in a straddle and lift one leg at a time with locked knees 15 times on each leg) and basic core and leg workouts will make a much bigger improvement than stretching alone. In my experience, hip flexor workouts alone will make a huge difference, but it would help to add in the other conditioning as well.
Also be sure they understand what they're doing and what is right vs. wrong and why. If they're doing workouts/stretching incorrectly it could double the time needed to make any noticeable improvements therefore wasting your time, their time, and making the parents impatient.
