Well, I can tell you we had the opposite problem, we started out as a party of 3 and went down to a party of 2, I had to call to change it because I didn't want a problem with getting my deposit back for the 3rd person (that wasn't going to be there).
I was told we may have to wait longer because we'd be waiting on a table that seated 2 instead of 3 or 4. We did wait a good 20 minutes or so and watched all of the parties that were there ahead of us and some after be seated before us (didn't make 4 yr old dd too happy, she didn't understand).
We did end up with a great table by the window though.
If I were you I'd call after you arrive at WDW and let them know you would like to join your dw and dd for breakfast, that when the PS was originally made you weren't planning to join them for the trip, they should be able to change it at that point as long as it's not a REALLY busy time of year (even though CRT's is busy all the time). But, I wouldn't wait til you get to the park that morning.
