Unlike chickens, ducks get their wing feathers almost last, not first. They get the feathers that will touch the water, like their chests, first. So, they are really big by the time they can fly.Ha. As fun as that visual is, I'd rather avoid it in reality!
The duck coop is at ground level below the chicken coop so that part shouldn't be an issue. It's just getting in and out of the run that they might have a tough time with. I think I'll go ahead and see how they do brooding in there and reassess whether they can manage the elevated run door as they grow. Worst comes to worst I can let them through the full height human door as needed until I get a chance to add a ground level duck door.
We brood in the coop with a Mama Heat Pad and the chicks are allowed out of the run (supervised at first) once they start flying well enough to get through the elevated door themselves. If the ducks can't get out I'll probably let them hang out in the run for a few more weeks to see if they catch up and then give them their own door.