If you have a way to contain them outside, they might enjoy some daytime ranging in the weeds to catch bugs and pluck grass. If you're home with them all day, you can just take them out and let them run around. They won't go far and will follow you around if you go slowly. If you don't have time to be outside with them, you can take the top of a dog crate, or hamster cage, or anything similar and set it down in the grass and put the babies inside. Of course, you don't want to leave while they're unprotected like that, but it will contain them so you can do chores while keeping an eye on them. If you have a pen set up for when they are grown, you can put the smaller cage inside the pen and let them run around that way. They'll love eating grass and weeds and digging for bugs and it's SOOOooo cute to watch them try to snatch flies and gnats out of the air.
At five days, if your weather is warm, they can go out without supplemental heat for an hour or so. If the weather is cool, you'll need to make sure there is a heat source available, or just take them out for very short trips and bring them in when they start to act cold.
If those are not options for you, then peas, algae (if you have access to a lake or other source), spinach, lettuce, even weeds from the yard are great chopped up in their water (the water keeps them fresh, and they love dabbling for treats). Clover, grass, dandelions, and plantain are good choices. If you're not sure about identification, look it up or avoid it--the ducks are unlikely to eat things that are poisonous to them, but it's better to be safe than sorry on that score. Nightshade, for instance, might harm them just from steeping in their water (though you're unlikely to mistakenly use it--the plant is covered in spiky fur and hurts to touch, plus it looks kind of sickly and dangerous--spidery potato-like leaves on a spindly frame--it's just an unpleasant plant altogether, though I've heard that the bright purple berries taste quite sweet right before they kill you).
Anyway, I've never noticed a difference from diet, but then I mostly raise mine outdoors in wire brooders now after the first couple of days. I live in a climate that allows that, and I'm grateful for it! I have noticed that the baking soda during those first couple days makes a HUGE difference. I still have to clean it out twice a day, but at least the stench is minimized.
Good luck!