I also agree, in this case I would not declaw. If your cat is already aggressive, when she realizes her first line of defense is gone she could become an extreme biter. She might become reclusive for the same reason. I've met several declawed cats and admittedly most are the same as they were before the surgery. But the factors I mentioned are a risk.
After speaking to a vet, I'd recommend calling your nearest cat rescue organization. These men and women are wizards at practically any cat. Many people that rescue and rehome cats have rehabilitated ferals in their home.
I don't think you should feel guilty over it. It happens and that's the way it goes at times. I hope it works out for you.
I may be a fairly new fish person but I know cats up and down. This behavior really is relatively common. Cats are strange creatures that flip out over the oddest reasons that takes outside influence and experience to figure out.
I'd suggest reading Pam Johnson-Bennet. She's a cat behaviorist and has written several books. It may not help your problem but it'll help you to know you're not alone.
Like I said, before you put the kitty down please call your local cat rescue. They'll be understanding, compassionate and I promise there will be a nice soul who has turned the worst offender into a great companion. It may take another home, medication, and a new environment but it has been done.
Have you tried Feli-way? It's a product, sort of a feline holistic xanax. You may also want to look for Bach's Rescue Remedy while you're trying to figure out what to do.
I wanted to add:

Msjinkxd is absolutely right. I wouldn't want my child to be scratched by an aggressive cat. It's too dangerous. An attacking cat can lead to stitches, especially on a little girl. Ask if your vet would mind housing your cat for a few days while you figure out what to do. Or put the cat in an unused bathroom with food, water and a litterbox.