Call the dining line and see if they can change your reservation to 5 people. Everyone needs to be included in all dining reservations regardless of how young or whether they will be eating or not because of fire code regulations. Tables at CRT are close together and the aisles need to be kept clear for the princesses and servers so you'll need a 6 top for your party. They can't just "throw a highchair at a 4 top."
If you show up with an extra person, CMs will do their best to seat everyone, but there is always a slim chance that they can't because the restaurant is at capacity or what we saw at a different DW restaurant - it was running far behind schedule so they would not seat anyone not on a reservation.
Keep checking online multiple times a day to see if a cancellation comes up. While there might be an uptick about 45 days before the day you want and close to the cancellation deadline, cancellations will come up at any time because of the severe financial penalty. If you are flexible about the day or meal you can expand your options.
You can also subscribe to the November cancellation thread:
http://www.disboards.com/showthread.php?t=3278305