A cat had 5 kittens yesterday, and 3 of them have 6 toes on both of their front paws. How did this happen? Their "thumb" morphed into 2 different toes!

It could be a product of inbreeding, which happens a lot with feral cats. After so many cycles of it the kittens tend to come out with deformities. Of course that's just one possibility, the circumstances of the kittens you're talking about could be completely different.

You have very special kittens :) Cats with more toes than normal are known as "Polydactyl" cats (poly meaning many, dactyl referring to digits/toes). Here's a great article with heaps of info on polydactyls.
One of our cats has extra claws on his hind feet. Apparently this is unusual if the front paws are normal. His front ones look completely normal, but he has two extra claws on each hind foot.