You can take a plastic container with a lid something about the size of a cool whip container place the lid on cut holes big enough for them to stick their heads into place this container on the tray you have now for your waterer. You’ll have to enlarge the holes as they grow but this works...
How precious. You have some smart ducks. As for immature lungs that I don’t know could be possible but only a vet could tell you for sure. Hopefully it’s something she’ll either grow out of or it’s just her and just normal for her. I can truly understand your being concerned but your doing...
Well like @CayugaJana mentioned Pekins can an do breathe with their mouths open and pant so this maybe just a normal thing with Pekins. Hopefully it’s just normal with your little one but just keep an eye on her. If she is eating good and interacting with her buddy more than likely she is fine.
@HappyDuckies. There’s plenty here who want to help can you answer some question please. What breed, temp in your brooder, and set up with pics please and what your feeding your ducklings. Pics or video of your duckling also if possible. We’re always happy to help but we need a starting point.