Hola, Angela! The easiest way to check availability at
Bibbidi Bobbidi Boutique is to call (407) WDW-STYLE or (407) 939-7895. If the location at Magic Kingdom is booked with budding princesses, you might have better luck securing a spot at the Downtown Disney location.
I haven't personally experienced the rides while dressed to the nines in princess finery. However, the wind on my face during Dumbo after my Gentlemen's Facial at the Grand Floridian felt surpassingly nice. Ah, who am I kidding? It was a mani-pedi.
All joking aside, when we treated our close friend's daughter to the BBB experience, she loved feeling like a princess while dressed up and decorated in the parks - especially in Fantasyland. She was 5 and she was in her element. Her little sister was almost 3 and loved being dressed up in the parks too (we did a little DIY princess treatment for her since she wasn't old enough to do BBB.)
If you can get a time, go for it. That goes for any gentlemen in your party at the Grand Floridian too.