Hi Catherine!
Oh wow! Your Walt Disney World Resort vacation sounds like it's going to be so much fun traveling with so many personal Princesses and Princes!
When it comes to
Advanced Dining Reservations, every body needs to be counted in the reservation number, even if they are a child sharing off of a plate or a baby who isn't eating solid foods yet. Your specific situation actually falls under two dining rules. The first is that children ages 3 and under are not required to purchase a
Disney Dining Plan, pay for a plate at a buffet, or be charged as attending a Character Meal. There is also no charge for this age and under at any regular Table Service meal or dessert event.
The second dining rule is should someone skip a seating for a reservation that was held with a
Credit Card Guarantee, there is no penalty as long as someone else from your reservation still shows. Disney is very understanding that things happen, whether due to illness or children getting super exhausted from their Disney Days. As long as part of your party still dines, even just one person, there is no charge for those who didn't. (Should your entire reservation no-show, the credit card that guaranteed the booking would be charged $10 for every person over three-years-old.)
Locations where payment is due in full at the time of the reservation (ex: Hoop-Dee-Doo Musical Revue) are a bit tricker as they've already been paid and there is usually no per-person refund within 24-72 hours of the meal. (Each location varies.) Should an adult decide to sit one of those out definitely notify Disney Dining right away at (407) 939-3463.
I hope this helped, and We'll See You Real Soon, Catherine!
~Kristen