DESPERATE DUCK HELP PLEASE

Hi there, I really need some advice please! We had a mallard make a nest in the garden and two weeks ago the eggs hatched. We have an enclosed walled garden and live right next to a very busy road so I contacted RSPB who advised us to wrap up mother, catch babies and put them in a box and take to the canal, which we did, in the general direction that she was flying to every day. We were surprised the day after when she and the drake returned, him flying off straight away, and she was absolutely starving so I gave her some of the special duck food I had bought. The thing is she is still coming every day and eating all the robin's food. Yesterday and today she has stayed an hour with no prompting from me. Does this mean the babies are dead, will they build a nest here next year and if so do we just leave them to it when they hatch and should I stop putting any bird food at all out? Thanks for reading, getting desperate here"