Don’t assume that ducks fall under the same rules as chickens. Definitely look further into the laws or bylaws for your area because there are definitely some places that have very clearly outlined rules for chicken hens only, and prohibit other types of poultry. Not to be a downer but it would be much better to find out now that they’re not legal, than after you spend a lot of time and resources setting everything up and having to rehome them after you’ve fallen in love with them.
My ducks took a bit of time to figure it out, but were able to use a door opening that had the a plastic flap that’s meant for dog houses. I should think they could eventually figure out an actual dog door as well. You still have to be aware of predators during the day, however, as not all of them will necessarily be nocturnal. Having a secure area you can put them in if you’re unable to watch them would be a good idea.
one other question is how many ducklings are we talking about? If stocking density is moderately high you will wind up with a pretty sparse, crater filled yard anytime you get precipitation and around their water area. Basically anywhere they can drill in the ground they do, and plant life will be trampled and eaten. The more birds in a space the faster they clear it, unfortunately.