Hello Rebecca! I love that you're taking your niece for her first Walt Disney World vacation - it will be a time that she remembers for the rest of her life! Unfortunately, the Bibbidi Bobbidi Boutique is strictly for guests ages 3 -12 years old, so your niece would not be able to get a makeover there. However, there is another alternative that she may enjoy.
The Pirates League in Magic Kingdom does an Empress makeover or a Mermaid makeover, and is available to guests of all ages! Click the link I've provided, and you will find a full description and prices for each of these fun makeovers. Just like Bibbidi Bobbidi Boutique, you'll want to make advance reservations as early as possible, up to 180 days in advance of your arrival at a Walt Disney World resort. You can call 407-939-2739 for more information or reservations.
Be sure to pick up a complimentary 1st Visit button for your niece to wear throughout her Walt Disney World vacation. Guests wearing these buttons often get special greetings from Cast Members or characters, and it's sure to add a special touch to her vacation. Just stop by Guest Relations at the front of any of the theme parks to pick one up.
Rebecca, I hope this helps with your planning. Please don't hesitate to write me back if you have any other questions I can help you with. Have a magical day!
Kristen