If the babies' eyes are closed, place blades of grass or very light twigs in a X pattern over the nest. If it is disturbed, you'll know Mom has returned. Baby rabbits are on their own when they are about the size of an adult human fist. The answers to these questions will determine the best course of action. If they do not, call your nearest wildlife rehabilitator.
Birds have a poorly developed sense of smell.
Handling by humans will not discourage the parents from resuming their care. Humans hanging around the area to watch will. Make your observations from a distance. Call your nearest wildlife rehabilitator. This is a brancher or a fledgling.