They'll need some form of heat until they grow feathers. In nature, when the mother is raising the young, if they get chilled she will warm them underneath her. We mimic that with a brooder that has a heat source at one end so the babies can go to it if they are chilled, which is more likely without a mother.
Newly hatched chicks and ducklings actually stay under their mother for a couple of days in most cases. They don't need food or water yet as they have plenty of reserves from absorbing the yolk sac just prior to hatching. It's a critical time for them as they get chilled very easily.
Hopefully the mother with babies will take these on. Some do, some don't. The best time to introduce new babies is usually at night when they are sleeping on their nest. I don't know if this will be an option for you.
Have you picked up duckling-specific food for them?