There is a link to the Stickies in blue text at the top of the Duck Forum. In that list is a Sticky on treats for ducklings and ducks - some good information there.
If they are getting outside, and have been noodling around in the dirt, they should be fine. Most ducklings that age (to my knowledge) are kept indoors all the time. Due to weather, mine were in the house for quite a while. So I gave them a tiny bit of grit. Once they were outside nomming in...
Have they had some chick grit or clean sand? They don't need much, but they need some or there can be very risky digestion problems. And do keep limiting the corn. It's loads of calories with little nutrition.
Breathe.
Don't keep the younger ones with the bigger ones just yet. They need their own space - you may be able to subdivide the brooder.
Many ducklings go through a stage of being scared to death of anything larger than they are. It is heartbreaking and it is not forever.
First, make...