She's adorable. How about just some knitted tubes and attach icord or single crochet strings to them-- to go in her coat sleeves? I can't even imagine how hard it would be to put her little thumbs in the thumb places on a pair of mittens!
AND, I doubt that they'd actually end up being in the little thumb holes anyway?
Measure her wrist, width of hand around, length of hand open.. and add a little to it.. add a ribbed cuff .. do a little lincreasing for the width of her hand part.. ??
THIS is what I made for a baby born last DEC.. and I didn't get to take any measurements- I just guessed and they worked fine.. of course, he's now too big to wear them this year.