The eggs can be candled now (after five-six days). Any eggs that are clear should be discarded. You don't say where you're from. Here in Wisconsin I would bring the ducklings in where it is warm; the hen too, if you have the room.
Sounds like our spring weather. Mallard ducklings are quite hardy. Check with local people; possibly the ducklings could be left with the hen without being brought in where it is warmer.
Edited by tbevans - 10/26/15 at 10:18am
The ones with just a line of blood or a blood ring ("circle of blood") have likely died very early in development, but it's always best to keep duck eggs until at least Day 10 of incubation before discarding. Sometimes they can surprise you and ones you thought weren't developing will have a nice red spider-web of healthy blood vessels the next time you candle them at 10 days. So, good call keeping your Mallard eggs for now. Good luck and keep us posted! :)