Help. Totally new to ducks. Have 3 ducklings
I'm so sorry to hear about your daughter.
The pekins, there are 3 of them and they are one week old?
Cardboard may be difficult to do as a brooder due to water, unless you have a barrier down to keep the water from soaking through the bedding and into the cardboard. You'll want to be sure the box is large enough that they have an area that is warmer and the rest cooler (room temp if they are inside).
You can pick them up, if possible, try to do it at their level and not from above scooping them up. You are 283923 times larger than them and they will just see you as something trying to capture and eat them. More time spent with them, the friendlier they will be. Picking them up will get them used to being handled.
As long as your pen is entirely predator proof from whatever you have in your area that likes to eat ducks, they should be fine in the pen until you get home each night. If 100% predator proof, you might not need to lock them into their house when you get home, but otherwise, lock them in their house in the pen when you get home each night and let them out again in the morning.
They will get used to your routine.
- Overrun with Runners
The main point is that everything likes to eat duck, and wherever they are, they will need to be safe, have water to drink, and some food to eat.
There are 3 gallon chick waterers that may work, and with ducks you'll need to have something under that to catch or absorb about 2.75 gallons of water. But they need the water, really. For waterfowl, it's critical.
If they are in a box, come up about a few inches above the level of the box. If you stand straight over them, that will make them more anxious than staying closer to the top of the box.
If you have a laundry room or an ok sized bathroom with easy clean flooring, lock the dogs out of that room and bring the ducklings in with you and sit on the floor while they run around.
at 3 weeks, I'm not sure if they'd follow you closely in the yard or not. Worth a try if you want, again, lock the dogs in the house first though.
But before I did that, I'd get them crazy about peas. So if they do decide to be independent outside, you can use peas to get them back to you to scoop up and bring back inside.
An options inside for getting them closer to you would be to put their box on a card table so there isn't so much of you going into the box to clean over them or pick them up or whatever.