My cat Felix had kittens with a stray cat?

The kittens sound fine and healthy, I don't doubt that they will survive. However, it's very, very important that you don't let them continue to live outside. Not only because it's a tough life to be a feral cat, but because these kittens will soon grow up and breed. A colony of cats grows very quickly, and there's already a big cat overpopulation problem. As somebody else suggested, find a place called a "no-kill" or "low-kill" shelter (they only put an animal to sleep if they're in severe pain and can't recover.) They will spay the mom and kittens so they won't keep reproducing, and likely find good homes for them all. I know it might be hard for you to do, but it really is the best for the animals involved. Good luck!