Amprolium is safe
Medicated feed used to be medicated with what they call 'sulfa' drugs, which were added to the feed in amounts that were good for chicks, BUT ducklings eat more than chicks do, so they would overdose themselves on the medication. Thus the myth that all medicated feed is unsafe for ducklings was born.
Those drugs are, for the most part, no longer used in feed today. Amprolium is totally safe. Now, they don't really need it, so if you can find a food without it it's better to feed a non-medicated because no sense in giving them a thiamine blocker that they don't need, but if you can't find a feed without it, it won't hurt them and you can feed it.
And just in case you don't want to just take my word for it, here's a quote from a veterinarian with Purina:
And here's a picture right out of Storey's Guide, who did testing with their own ducklings:
X2, feeds with amprolium are fine for all waterfowl, but if they are chick feed, they don't have enough of other vitamins, minerals, and amino acids that waterfowl need.
Might want to add niacin - brewers yeast or nutritional yeast as the chick grower won't have enough for your ducklings which by the way they are so cute!
X2 on all this!!
But if you switch to Purina Flock Raiser, it *should* have everything your ducklings need, without additives... I would still get Brewers Yeast/nutritional yeast (NOT baking yeast) to have on hand *just in case*... a few times the formula for the flock raiser wasn't correct and ducklings had issues...