Give them a call on 0871 231 4787.
You'll need your flight number & booking reference available, hopefully they will be able to help or best advise. All seats are allocated 24 hours before and a seat plan is sent to check-in for the flight which they don't have much flexibility for changes etc, so best to contact them now I'd say. Your seats on the Santa flight will be the same inbound as they were outbound (at least they should be).
Enjoy it's a fantastic and magical place to be!
Hope that helps.