I'd go with whatever your daughter feels comfy in, and is stable on her head. Yes, do check it's current RS standard. Some BSI KItemarked hats are still legal and safe, but not to a level for competition or riding schools.
And a word of warning - don't always believe fitting advice. I bought a new hat recently, and was measured for size by a qualified fitter. I was a bit surprised I was in a smaller size than my usual, but trusted the advice given. The hat I bought is too small, and gives me a bad headache after 20 minutes or so. An expensive lesson. So always err on the side of comfort!