Hi Claire,
Officially, Disney's complimentary transportation is available one hour prior to Disney Theme Park opening time up to one hour after Disney Theme Park closing time.
That said, the folks at Disney are aware that many Guests staying at Disney Resort hotels will have made early morning breakfast/special experience reservations and I can honestly say that my family have never had any problems using Disney's complimentary transportation to get us to the Disney Parks as early as 7.00am. I would suggest that you be at the bus stop at your Resort hotel at least an hour prior to the time of your reservation at the
Bibbidi Bobbidi Boutique.
Your Daughters princess transformation will take 30-60 minutes so I would recommend that you leave 90 minutes between the time of your reservation at the Bibbidi Bobbidi Boutique and your breakfast reservation at
Cinderella's Royal Table just to be on the safe side.
If you have any concerns about transportation at all, I would recommend that you stop and speak to a Cast Member at Guest Services at your Resort hotel the day before your reservation for confirmation that buses will be running early enough for you to get there in time.
Wishing you a magical trip filled with memories which will last a lifetime!