I shop thrift stores often and very much doubt that you'll find a dress let alone one that fits the period. I wouldn't spend the time looking.
You can make a skirt easily. As OneLittleBigBoy describes. I suggest you could pair the skirt with anything, even a T-shirt if you add a white cardigan. I have seen white cardigan's for girls at Target and Sears and Macy's but it's been awhile. I'd call around.
I don't know about printing cherries on fabric so that it's an overall pattern. How would you do that? I suppose you could make a stamp and stamp it but that sounds like too much work.
Ankle socks with mary jane or similar shoes would work.
You could go to a fabric store and look for blue fabric with a small red and white print and then ask for the names of women who sew and hire them to make the dress. A seamstress would know how to modify a pattern to make it look like that era. It's the small print, colors and the cardigan that makes this costume Emily's.
A gathered skirt and blouse with a Peter Pan (rounded collar) and cardigan would work. It wouldn't have to be a print. Does she have an Emily doll she could carry?