Just a quick update: Though she wouldn't keep the crown on her head (we did get a few pics I'll add later) it fit her PERFECTLY!! I was so proud!! I was using my 4 1/2 year old son as a pattern size, and guessing how much smaller it needed to be for her.
All of the older girls at the party (and some of the boys) kept popping it back on her head every time she took it off. Proud Auntie moment!!