Congratulations on getting those difficult-to-come-by Royal Table reservations!
If you check out this handy guide to the Disney Dining Plan found
HERE, you'll see that the Princess Storybook meals at the Akershus require only one meal credit. You will see some of the same characters at both of these meals. Often Belle, Sleeping Beauty, and Snow White can be seen at both the Royal Table and the Akershus. However, Ariel typically only appears at the Akershus. As to whether you should do both meals, the choice is obviously up to you. I will say that when my girls were preschoolers, they would have been thrilled to dine with the princesses at breakfast, lunch, and dinner during every day of our vacations.
If you do decide to dine at both of these meals, I have tip to help make things even more special for your girls. When you see one of the princesses that you've previously met, you should start the conversation by saying something like, "Oh Snow White, how nice to see you
again." This clues the princess in that she's seen your daughters before so she won't ask some of the basic, "What's your name princess?" questions that might interrupt the magic.