Hi Jill,
Cinderella's Happily Ever After Dinner at
1900 Park Fare is Fabulous! I think you will have a great time there!
Unfortunately, however, if your dinner reservation is at 6:00, I don't think you will be able to make it back to to the Magic Kingdom at 7:00.
For Table Service meals, I usually plan on being in the restaurant from an hour to an hour and a half. If your reservation is at 6:00, you may not be seated until a little after that. You would still have to catch the monorail back to the Magic Kingdom, go through security, and walk onto Main Street.
However, I do think you can get back in time for Wishes, which will be at 8:00 that night. But keep in mind, that you can also watch Wishes from the Grand Floridian if you don't want to feel rushed. If you want to do that, walk down to the beach area or the boat dock. Outside Gasparilla Grill is another great place to watch the fireworks from the Grand Floridian.
Have a magical vacation!