Hi, welcome to BYC! wood ducklings need live food to start them out, small meal worms or tiny red worms chopped up. How old do you think they are? Good luck with your ducklings.
If you have a photo of the beaks close up you can tell by the nail on the end of the beak. If they are Mallards, there shouldn't be a problem feeding. You can also sprinkle crumbles of food on their backs to prompt them to eat.