Hello Rita! Welcome to the Disney Parks Moms Panel.
When my family participated in
Mickey's Not-So-Scary Halloween Party this last August, we booked Magical Makeovers for my son and daughter at
Bibbidi Bobbidi Boutique - Magic Kingdom to accentuate the costumes we already had on hand. My daughter became "Princess Stitch" and my son became a glittery Kylo Ren (knight).
In our circumstances, my daughter insisted on wearing her costume, we stayed relatively close to Magic Kingdom at
Disney's Polynesian Village Resort and the costumes were not uncomfortable. In order for you to make the best decision, I recommend you make this decision based on desire, convenience and comfort.
If your child really wants to wear the Disney Princess costume you already have and will not be too uncomfortable for your child or yourself, then I would recommend it. You will find many Disney Princesses throughout Magic Kingdom and your child will fit right in.
If bringing another outfit or lugging around a costume sounds like a big inconvenience, then I recommend that your child wears it to Magic Kingdom.
If you feel your child would not be comfortable dressed as a Disney Princess for the length of time you will be in the Theme Park, you are already bringing a Theme Park bag and you wouldn't mind the extra carrying, then I recommend you bring a change of clothes and make the decision as the day wears on.
Have a Magical Makeover!
~Todd