I hope you have a large pond or lake or something nearby, where they came from?
Wild Mallards often leave a larger flock area to have some private time for mating. Sometimes the hen will still choose a different area for nesting so they may both leave your yard.
If she choses to nest in your yard you will see that the male usually doesn't hang around, but will instead go back to the main flock area. Incubation is about 30 days. You could put out some corn or other waterfowl feed you can get from a feed store for her and water, not breed.
A few days after hatching the hen will usually leave the nesting area and lead the small ducklings back to the main flock's area since they need access to a large body of water for safety and the eating of water plants and bugs etc. Obviously the little ducklings can't fly and at that point neither can the adults, since summer is the time they are molting their old feathers and growing in new ones. This makes them unfortunating very vulnerable to predators and cars for months.
That is the only point at which you could or should intervein. If you know their path back to a pond or lake is too dangerous... then you could carefully catch them all and put them in a box and deliver them safely.
The ducklings grow very fast, and by late fall they are capable of flight and will migrate away with their parents.