I like Margaret Mildred best, but either of your names are nice. I'd probably pick Francis because it makes a more interesting sound; otherwise you have three hard "c" sounds all in a row for her name (with a syllable in-between).

I'm guessing Cassidy would be an unusual name in 1924-5 (when Kit must have been born) and might very well have a much greater association with Butch Cassidy than it has today, since Westerns were more in vogue. Not that that's a bad thing, but just something you might consider.