Hi Alberto,
Officially, Disney's complimentary transportation is available one hour prior to Disney Theme Park opening time and up to one hour after Disney Theme Park closing time. On days when there are Extra Magic Hours, Disney's complimentary transportation will begin running one hour prior to the the start of the Extra Magic Hours.
That said, the folks at Disney are aware that many guests staying on-site will have made early morning breakfast reservations and the Disney Buses should be up and running in plenty of time to get you there.
My family have recently returned from a 7-night stay at Walt Disney World and frequently saw buses heading to all four of the Disney Theme Parks as early as 7.30am.
I always recommend allowing at least an hour when travelling from one place to another at Walt Disney World and so would suggest that you be at the bus stop at Disney's Art of Animation Resort for one hour prior to the time of any dining reservations.
If you have any concerns at all, speak to a Cast Member at Guest Services when you arrive at your Resort hotel who will be able to confirm that Disney buses will be operating early enough.
Wishing you a magical trip filled with memories which will last a lifetime!