It is important to make sure they warm up before they stretch. Jumping on the trampoline or doing star and tuck jumps on the sport for a few minutes first will do the trick.
If they start practicing the splits without this warm up they won't be able to stretch to their full capacity, so their flexibility won't increase. Also there is a greater risk of injury. After a warm bath or shower is also a better time to stretch.
Do a general all over body stretch before going into splits, as well as some pre splits stretches like lunges and quad stretches. This is important to loosen, the muscles first before doing the splits.
I would do 2 minutes each leg and then repeat all legs for another 2 minutes. If they are not naturally flexible they will need to hold their splits longer to increase their flexibility.
Both dd's are doing the splits along with other conditioning exercises and have only been doing it for a few days and holding the splits for 1 minuet was recommended on the conditioning form, I can increase it to more than 1 minuet, my youngest dd held her splits for 1 minuet and 22 seconds. In terms of flexibility my youngest is the most flexible one and my oldest is not and needs a lot of work to build her flexibility up.
The conditioning exercises that were given to do over the Christmas shutdown are:
Skipping - to warm up
Holding front support - start at 30 seconds building to 1 minuet
Dish and Arch raises in a controlled way - for 30 seconds building to 1 minuet
Triceps Dips using a chair - for 30 seconds building to 1 minuet
Squats - for 30 seconds building to 1 minuet
The Plank - hold for 30 seconds building to 1 minuet
Japana (Pancake) Swim Through - 5 times, my dd's like to do more of this one
Bridge hold for 10 seconds - do this 3 times
Right and Left Splits - Hold for 1 minuet
Box Splits - Hold for 1 minuet (both dd's struggle with this one, youngest is better at it than older dd, would you recommend doing this one against a wall or sofa? At the moment both dd's are using cushions and laying forward like in pancake and holding it.