Hi Sabrina! Little Olivia is going to love her princess makeover! My daughter has made a few trips to the
Bibbidi Bobbidi Boutique, and each time I make the reservation, the castmember tells me to bring her own brush or comb. However, every time we go to the Bibbidi Bobbidi Boutique, the Fairy Godmother-in-Training never asks for the brush we've brought with us; instead, she uses a plastic comb, which she then gives to us to take home along with the makeup she used. For this reason, I don't think it's crucial to bring your own brush, but you should definitely make sure your daughter's hair is free of tangles so the Fairy Godmother-in-Training can easily use her hands and comb to style her hair.
Bringing your own dress to the Bibbidi Bobbidi Boutique is a great idea and really helps to keep the cost down. I'd recommend that Olivia put on her dress prior to her appointment. That way she won't mess up her cute new princess hairdo trying to pull a t-shirt over her head so she can don her gown and complete her princess transformation.
Hope your little princess has a royally good time!