It is difficult to say for sure, but ducklings need water and food 24/7 for their first couple of months. Some say six weeks, some say ten, but about two months. I keep food with my ducks all the time for a number of reasons, as do many people. Some feed adults twice a day, but that's for adults.
It keeps the ducklings a little more calm, I think, to always have food at hand, and they don't gorge themselves.
Just make sure they have plenty of water at this point and keep an eye on them.
If they are breathing okay, pooping, otherwise acting normal, I think you just have some "puppy belly." And, literally, it will pass.
Please do keep food with them all the time for a while yet. I have found that ducklings and ducks, at least mine, don't read the books, so I went with my gut on when to do things differently, if at all, and I watched closely at first to see if it was working. An example was bedding. We started on old towels, and that was labor intensive, so I tried shavings. Har, had, har. Yes, they DO eat shavings!!!
Back to towels, then tried shavings a couple of weeks later only to find I am allergic to them. sigh.
Just illustrates that adjustments need to be made with these beautiful creatures.
When you have time, tell us more about the mallards!
I forget what the occasion was, but one day I saw Sieben (one of my runners) with a pouch on her bosom, she must have just scarfed down a bunch of food, and it looked so strange! I was about ready to take her to the vet.