Adorable ducklings! I think it would be okay to leave them with your hen--broody hens usually make good mothers, even with non-chicken babies. However, just to be sure, I would post your questions in either the Raising Chicks section of the forums, or the Duck section of the forums.
I've never had broody hens raise ducklings, but I've read that they can mother ducklings and even goslings. Still, it would be a good idea to post this in the Ducks section, as those people are more knowledgeable on ducks.
Welcome to BYC. Glad you decided to join our flock. Congratulations on your hatch. Ducklings are the ultimate in cute. Your mother hen should do a good job taking care of the ducklings. Of course, she is going to have a panic attack the first time they go in the water. ) Please feel free to ask any questions you may have. We are here to help in any way we can. Good luck with your ducklings.