Hi Laurie,
You have several options for reaching Cinderella's Happily Ever After Dinner at
1900 Park Fare in the Grand Floridian resort. If you are coming from your resort, driving would be your fastest option. It will take you 15-20 minutes to reach the resort by car. I would allow 30-45 minutes total so that you aren't rushed finding a parking spot and getting to the restaurant.
If you want to use the free Disney transportation, you can catch the bus at Art of Animation to Magic Kingdom. From there, it's a short monorail ride to the Grand Floridian. The bus ride takes about 20 minutes, but you need to build in possible wait times for both the bus and the monorail,so I would all at least an hour for travel time, maybe a bit more.
If you happen to be in one of the theme parks that day and not leaving from your resort, you can simply catch the Disney transportation from there to the Grand Floridian. I would allow 15 to 20 minutes from Magic Kingdom, 45 minutes to an hour from the other parks.
I always think it is better to give yourself plenty of time to reach your dining destination. The restaurants are generally forgiving if you are a few minutes late, but who wants to arrive at a meal stressed and flustered from the mad dash?
It sounds as if you have many great experiences planned for your family. Please let us know if there's anything else we can do to help. Enjoy!
Wilma