Are you feeding them a commercial food? That should have all the proper nutrients. It doesn't always happen. We had a probem with a commercial food about six years ago and lost all but two ducklings out of a dozen and most of our cornish cross chickens. If you're feeding a commercial food you should ask the seller if other people are having this problem.

I'd stop giving them the oatmeal so that they eat more nutritious food. It's not something a duck would eat left to its own devices. They wouldn't eat hard boiled eggs either, but they're at least getting protein from the eggs.

There are vitamins you can mix in their drinking water. I think you need it immediately. Your feed store should have it. It's a powder, will have mixing instructions on the bag, and is very simple to do. If the feed store is open on Sundays I'd go today. Good luck.

Keep up with the greens. Some oatmeal is okay too. Make sure they have access to grit (fine)and plenty of drinking water. I'd also feed them 'duck & goose' grower feed. Do not feed them the following types of feed: turkey, layer, broiler, gamebird or medicated chicken feed. You can substitute with a chicken grower feed. Add 1-2 cups of brewer's yeast to 10 lbs. of feed for the extra B vitamins ducklings need. Also make sure you aren't using newspapers for bedding.