I have been reading up a bit about my new little charges & apparently the mother can make her nest up to a couple of miles away from the nearest water source. When the ducklings hatch mother waits until their feathers are dry & then heads off with her tiny family to the water. Once in the water the ducklings are quite self sufficient, but obviously that journey can be very perilous! The babies cannot fend for themselves unless they reach water so I feel vindicated in rescuing them.
Although the ducklings (named Ibbity, Bibbity, Bobbity, Boo!) will not have the most natural of upbringings they will certainly be safe & sound.
PS. They are gobbling up their chick crumbs & doing very well. I'm sure they have grown already!