Children can adapt to sleeping with a cast on, especially the younger ones. Initially, the child will be restless and sleep for short periods. They may find the cast itchy or they may find it difficult to turn in bed. For this reason, they may need to be attended to turn them over and put back to sleep.
The cast can act as another layer of warm clothes and the child may overheat at night. Use less bed clothes. Some find that a warm rug under the child may be enough. Again, use plenty of pillows to support the lower back and ankles.