Place yarn or a twig on the nest. The mother is coming back if the yarn or twig is moved. Check the bunnies to see that they are warm and hydrated, it may be your only way of knowing the rabbit has been to the nest. If a bunny is not cold, is fully furred, eyes open, and not in danger from pets, it has a better chance on it's own than with human care. If you have to chase it, it doesn't need to be rescued.
Improper food causes diarrhea and death.
Treat as an injured animal. Place papers or ravel free cloth in the bottom. Get and keep the body temperature warm using a heating pad under half the box so the animal may choose. A hot water bottle or jar or warm water wrapped in a ravel free cloth will also provide warmth. Stress is the biggest killer, so keep the animal away from people and noise.