Thanks so much for all of the responses. Annie, thanks for telling me your story -- I'm sorry for your loss, too.
It really helps to hear other people's stories and know it isn't just me. (I felt -- still feel really -- like such a terrible chicken mama... how could I have let this happen? I should have known better, I should have protected them more, heck, I should have camped there all night... But it's good to know that it's actually pretty hard to protect babies from predators, and everybody makes mistakes.)
BUT -- REALLY GOOD NEWS!! This morning, by some miracle, a neighbor found one of my chicks, a little RIR. (Of course, all I found was the wing of my favorite chick, and while I love the little RIR, she wasn't one of the ones that I had named and knew real well...) But I am still unbelievable grateful. The RIR was at least 200 yards away from the coop. Somehow she survived the fox attack, survived the night (full of owls and raccoons), survived the next day (full of hawks) and then survived an entire second night by herself. I don't know how she made it, really, or how she ended up being found. But I'm glad she did.
She doesn't seem to have any bite marks on her and she's happy and active, so I put her in with the rest of her sisters, where's she's scratching and scrabbling for treats with the rest of them.
The other, bitten bird is still more or less the same. She's eating, drinking, and pooping, but is a little unstable when she perches on my hand. She's got antibiotics in her water, which will hopefully prevent infection... I called the vet's office and they said fox bites are real bad for infections. (I couldn't bring myself to spend $80 to visit the vet and get prescribed antibiotic drops, but I did get $7 antibiotics from the feed store and mixed it in with her water.)