I've had this twice - one was born with frog legs and, yes, it came right within a couple of weeks (and she was the best show cat I have bred to date). The second wasn't frog legs but might meet your description of splayed legs - back legs right out at the sides making it difficult for the kitten to walk. That developed at about 4 weeks and took about a month to right itself as I recall, the cause in the end was really simple, he had a HUGE fat tummy. In the end when he grew the tummy went down - in fact he became the thinnest of the lot - and the legs went back into their normal place. He was fine by the time he was sold.

Liz