Have you got a local park? Perhaps with a pond? There's a lake out by my place, and people feed ducks there--nothing ever bothers them. Most of them are pretty tame, too. Ducks are very self-sufficient. Get them introduced to water early--we have two pekins that won't go in our pond because they're frightened of it--but keep an eye on them, because despite popular belief, ducklings can drown. Teach them to stay around water so that they don't do stupid things like leave the water and wander towards a road. One thing you could do is buy floating fish food and teach them to skim it off of the surface of the water.
Speaking as someone who's spent a lot of time around mallards, they should be fine unless you introduce them somewhere with a huge predator population. In a pond or on a lake, they feed themselves very well. Release them as soon as they're fully feathered and when the weather's mild. The younger you release them, the better they'll adjust. Before that, teach them to hunt snails and bugs and such--or just stop off every day and throw them some corn until they get the hang of it.
Even if they have imprinted, baby ducks have to leave the nest eventually--they don't stay with momma forever.
If you're really concerned about them, find some farmer who wants duck eggs and has a pond. That should work out well for all concerned.
