I'm so sorry for your loss!!
I lost all but one of my original flock to a fox. They were in an enclosure, but I underestimated a fox's strength and it pushed right on in. So you're not alone, and you really shouldn't beat yourself up about it. It happens, and now you know. Your girls will be okay, too. I think it's healthy for children to learn to grieve for a pet, as it prepares them for the realities of life which, unfortunately, include death at times. I'm also a proponent of letting children see the bodies if they choose to do so, as I think it helps them process the death.
As for the electric fence hurting them, it will hurt a lot. But it will not injure them if it's installed correctly (i.e., following the manufacturer's instructions). I installed electric fencing for my goats last year, and goat fencing uses a very high voltage. I tried to keep my then one-year-old away from the fence, but he got to it one afternoon while I was feeding my ducks, and the first I knew was when he started screaming. Because of the way the electricity travels through the body, it doesn't just hurt the hands that are touching the fence, so he didn't instinctively know to let go--he was still holding on. I ran over, removed him (I must have grabbed him between pulses because I didn't get hurt myself--the electricity will travel through as many bodies as are connected to whoever is connected to the fence), and he was fine. He is two now, and he knows to steer well clear of that fence, but he sustained no injuries from the incident. It's not how I would have preferred to find out that the fence won't injure a small child, but it was effective. The literature that comes with the fence will tell you that it's important not to let their *heads* come in contact with the fence. Apparently, it can be harmful (even potentially fatal) if a child is on all fours (i.e., lots of contact with the ground) and puts their head against the fence. As far as I know, there is only one known instance of that killing a child, but it's enough that the fencing always carries that warning. If your girls are above the age of crawling, it shouldn't be an issue.
Also, of course, if they're old enough you can teach them verbally to stay away.
Anyway--you've gotten really good ideas here. Everyone does it a little differently. With just one duck, you could easily just shut her into a dog house at night, with the opening securely closed. If she's full grown, she can be fine without food and water overnight. I use a chain link dog kennel, with hardware cloth around the bottom 18" to prevent raccoons reaching through and eating their heads off (which they will do--I learned the hard way). I installed the kennel over wire fencing laid on the ground and topped with mulch. Then I covered the top with more wire.
However, if I had it to do again, I'd probably skip the top & bottom wire and go with the electric tape close to the ground and maybe another strand at chest height or so.
Of course, you're going to need to find your baby a companion as quickly as you can. She'll be terribly lonely until you do. When I had just the one survivor, she didn't eat for several days, and then she would only eat if we were standing right beside her. It took me a while to find her some companions, and she became very attached to us in the meantime. We had to hand feed her or she would just ignore the food. Once she had companions, she remained friendly toward us but went back to being a normal duck with normal ducky ways. Anyway, if you mention your location, probably someone here on these boards is near you and might have a duck or two to spare.
Good luck!