In summer, I put ducklings outdoors as soon as they are two days old. They go in a wire-bottomed brooder to control water mess and stench. Half the wire is covered with straw to protect their feet and insulate against cold drafts. The other half is left open and that's where their water goes. In the half with straw, there are TWO 40-watt heat lamps, lowered enough to keep it very cozy at night. This works just fine for me. If your nights are only getting to 70 degrees, that's as warm as many peoples's houses. Just make sure they have supplemental heat, and I suggest TWO lamps in case one burns out--you won't be close to hear them peeping in alarm when they get cold, so make sure you have back-up. Also make sure, whatever arrangement you have, that they can't spill their water into the bedding to the point that it gets soaked and chills them that way. Again, since you won't be nearby to hear them if they are making alarm calls, it's important to be extra careful in these regards.
And, of course, make sure you locate and defend them against predators, which are many, even in urban and suburban areas (raccoons are particularly dangerous to little ones--they are strong, smart, dexterous, and hungry. They will open latches, tear chicken wire, and climb through small spaces for a scrumptious duckling dinner. They can also pull body parts through relatively small gauge openings and eat them alive. True story, too gruesome to detail here. Make sure wire openings are 1/2" or less, and latches are either locked with key or combo, or require a fair amount of strength to open). Our wire-bottomed brooders have chicken wire installed about eight inches BELOW the hardware wire mesh, to prevent a raccoon from easily grabbing a toe through the mesh (this happened to a quail once--tore its leg off). The extra layer of wire may be overkill, but it's peace of mind for us. I solid-material tray installed below the mesh would have the same effect.
Good luck. I understand why people like to keep them indoors, but I am one of those who likes the smell and mess to be OUTDOORS (and a wire-bottomed brooder is just easier to clean!). So I get it completely.