What if the mother bird leaves its babies?

It depends on the length of time. The duck will leave from time to time to feed or to continue mating (a mallard can hatch ten or more ducklings at a time). Before she vacates the nest, she will cover them heavily with feathers to provide insulation to keep the eggs warm and to keep them camoflaged from view until her return. Once she starts sitting, if she vacates the nest without covering the eggs, it most likely means she was scared off the nest by a predator, in which case she will need to return soon or lose her brood.