My 10 yr. old who thinks she's a teenager sometimes decided to wear a sparkeling purple crown to CRT during our first WDW trip. She wore it with a pretty sundress.
The princesses complimented her on it and she was delighted. I was thrilled to see her so caught up in the magic that she could be a little girl again (and in public!). It is one of my favorite memories. She changed clothes and took off the crown afterward and went back to being "cool". Luckily, I have lots of photos to prove it happened.
So...let her dress up if she wants to. Our little girls will soon be grown!