You need somewhere safe for mum and ducklings, including a house to shut them in overnight and a run. Can you leave them where they are and accommodate dad somewhere else, or divide off the run?
When I have ducklings with mum, I put in a gravel or holeless seed tray full of water, and then a flat stone in the tray and one immediately outside it, so that they can get in and out. Ducklings brooded by mum get a bit of waterproofing from her feathers, so they can swim in shallow water ok.
The chick crumbs you use for ducklings must have no ACS or coccidiostat in them. Most do, cos chicken chicks need it. The Smallholder range of chick crumbs is ACS-free and there will be others, but the common makes, ie Farmgate and similar, contain ACS which can make duckings ill or even kill them.
Sounds like mum is doing a good job of protecting her babies, can you get close enough for a pic?
