It's not The Hundred. The story starts out on another planet (Mars?). Details that I remember:
- Humans had left earth and gone to another planet after some unnamed Earth apocalypse, thinking no humans remained on Earth.
- The kids chosen to return to Earth may have been orphans.
- I remember the kids' oxygen source being pressed upon them if they forgot to return to their pod fast enough.
- There were no pets on the other planet, but they had video (I say video, because it was in the future, but you could say "film," as well) of cats, and when the young protagonist gets to earth, he calls a bird a "cat thing".
- Everyone on the other planet was dark-haired.
- Unexpectedly, Earth had survivors.
- The survivors may have lived by the sea.
- The kids wanted to stay on Earth, but the adults on the other planet didn't want that.
More details: It's a novel and I'm not sure. Likely pre-1975. Am thinking it might be Ireland but am really not sure. My friend who's asking said she also remembered that after being forced to use breathing apparatus on the other planet, the main character liked breathing the air of earth. I don't recall any alien or supernatural or extra powers. Everyone was human.
Does this sound at all familiar to anyone?