Ignore them and let them get on with it, they've got it sorted and know what they're doing. The drake does bother them when they come off their nest for a break but that's drakes, they will come off for about 20mins when they feel like it, sometimes twice a day if they're fed up. Just make sure there is food out for them so that when they do come off they can have some. Don't hassle them and don't keep an eye on them or inspect them etc and don't get too edgy when you think they should have had their ducklings by now - it will all happen. Relax, Muscovies are chilled. Ours do appreciate some wet brown bread - a whole slice to themselves if I spot them on their break - they do like to guzzle then. They usually hear when I'm putting the food out and come off for their breaks then.
Just watch you don't have herons or otters - we do have to put our ducklings and mums in a coop for the first few weeks or the herons pick off the young. Don't expect all the ducklings to survive. Our ducklings don't eat much for the first 3 days then they LOVE brown bread mashed up in milk or water, gives them a good start. We never feed duck crumbs just milky bread which they love for the rest of their lives.
Sometimes when they come off for a break the other ducks can get a bit annoyed with them because they act a bit strange and bickering can break out. And sometimes when they have ducklings they can be a bit over-protective but they all sort it out and it's not usually a problem. Its not like scrapping hens that want to kill the others offspring. They don't move around with their ducklings much for the first week or so, they usually stay safe. Good mums and great ducks, enjoy