From a book I wrote for beginning teachers, here are a few suggestions -- simple yet effective. These are for ages 4-6:
8. Butterfly Wings. With the bottom of the feet together (in “frog” position), move the knees up and down. Start slowly and build in speed and number. Can count out loud.
Uses: technique, stretch, imagination
Teaches: rhythmically structured exercise
Add: a stretch over, so that the nose touches the big toes; twist over to the side so that the chin touches the knee; repeat from side to side.
9. Hide Like a Turtle. (Dévelopée and extension). The legs and knees are together with the feet pointed (good toes!), the arms being wrapped around the legs and the head down and “hiding.” Then, have them “look up out of their shells” and have the hands go to the floor beside them. Each leg is then extended all the way, straight, one at a time, repeating the same leg. Then try BOTH legs together twice, and on the third time, have the hold the legs the try to balance with their arms off of the floor à la seconde. This is a very fun game and children seem to like doing it every time.
Uses: technique, rhythm
Teaches: balance, dévelopée, trying to achieve a goal by doing their best
Hint: fall over backward yourself on occasion, so they know it is “okay” to do so themselves and that it all right not to be able to achieve the end goal immediately.
10. “Frog” Position Sit-Ups. In the “frog” or “butterfly” position, have them reach their arms forward and then roll down and up, 4-8 times.
Stretching and reaching for their toes and feet with their legs extended out in front of them is always effective.
I might suggest you made up a floor stretch routine that they do regularly as a part of their class. When they learn to limber and warm themselves up as part of their regular training, then as they get older and more advanced, they'll often just do warm ups on their own without hardly even having to think about it or being asked.
Also, ballet exercises, in and of themselves, will limber and align them. I wouldn't be too overly worried if they are not little rubber bands right away. Flexibility and strength and control to go with it are built over time.
Best wishes for happy teaching!