Susan,
The good news is that the Dining Plan and making an Advanced Dining Reservation have nothing to do with each other, so no worries there!
I think that while
Cinderella's Royal Table may be the most popular meal in all of Walt Disney World, I prefer the
Akershus Royal Banquet Hall in Epcot for the Princess experience.
Why?
Well there are usually more princesses in attendance, thus increasing your chances of having Ariel there.
Secondly, I prefer the food, which is great. If your kids are not into trying new things, and prefer American standards, then steer for earlier in the day. Breakfast is more American fare, less Norwegian than at lunch. Lunch is more American/less Norwegian than at dinner.
Third, I like that there is that flexibility of having the option of breakfast, lunch, or dinner. That way you have a better chance of having it meet with your schedule needs.
You can look at the menus of each on the links that I put in this reply, so scan them to see which you prefer. And send me any follow-up questions you may have!
Have a magical (and delicious) vacation!