Hi, Dale! If a meal with the princesses is a must do for your family, I would be inclined to keep the reservation, but keep checking to see if something else opens up.
If you get to Park Fare a little bit early, there is always a chance you will be in your seat by 6:00 or 6:15, and then you can get through your meal, and head right to the monorail to head into the Magic Kingdom. You are likely going to miss at least the first 30 minutes or so of the party, but that leaves a lot of evening left!
If dining with the princesses was really important to someone in my family, I would probably do this very thing. It would be a great way to start off a festive evening! But again, you can check back often to see if something opens up on a different evening.
Have a great vacation!