Some children just have extremely tight muscles. Stretching helps, but if they don't stretch twice a day, every day, it is like having to start over. Both of my gymmies have suffered from muscle cramps at night since they were babies... and I can sympathize because I have had every muscle in my leg cramp AT THE SAME TIME in my sleep and I thought my leg was going to be ripped apart! I stretch regularly to try to avoid the leg cramps, but they still happen 3-4 nights a week.
When my gymmies are doing their splits in practice, they don't have the amount of time they would need (being tighter that the other girls) to completely stretch the muscles, but they still have to hold their splits just as long as the other girls. YG's hamstrings are so tight that she CANNOT straighten her front leg right now in her "splits" if that's what she is calling them.
In about a month - month in a half, I will be able to see her more often and we can work those muscles back out and get her splits back down.