Dear Laura,
Thanks for reaching out to us here at the Disney Parks Moms Panel! From your question, I am assuming that you're talking about the
Bibbidi Bobbidi Boutique makeover packages for children ages three to twelve? This is such a wonderful experience for little ones, and they just feel so terrific both while they are there and then when they are walking around the Parks for the rest of the day!
There are three package levels available and it sounds like you are leaning towards the Courtyard Package, which includes the tutu and t-shirt, sash, makeup and hairstyling, nail polish, and the Bibbidi Bobbidi Boutique bag. She definitely can change into the t-shirt and tutu at the shop (be sure to change first before her hair is being done) and wear it out into the Parks all day.
Also, you absolutely can bring your own dress or other outfit and she can change
into it there in the dressing room. They are very warm there at the store about what each individual child wants to wear and what parents would like to choose. When we have been there, there has never been any pressure at all to sell anything to parents who want to choose the less expensive packages.
Hope that your little one has a pixie-dusted memory to last a lifetime!
-Jan K.