I'm new to ducks too and mine are now five weeks old. At four weeks they were already starting to pant in with the chicks so I moved them to their own coop without a heat lamp and they do great. Started them off with a smaller bowl (size one duck can fit in) and they now have a large kiddy pool and all four swim in it and are loving it. I put them outside when it was 30 degrees (it was very sunny and warmed up to 55 throughout the day) and they were fine. They sat in the sun until it warmed up and then they tried out the pool I put them back in their coop without their food/water (to help save on messes) and put them back outside at 7am. We put them to bed about 8:30pm. You should post some pictures of your babies BTW: I was amazed at how fast they grew too!
ETA: I forgot to mention I was amazed at how little feed they eat now that they have LOTS of grass and bugs to eat. They are GREAT foragers!
What breed are they?