While I think it's possible that muster stations can change -- I think it is highly unlikely to be done on request by any given traveling party. Muster stations are based on occupancy - both stateroom occupancy and lifeboat occupancy - as well as location and route from the stateroom. Unlikely to modify just for one party/one time. The adults in your party will need to split as booked, and be responsible for the child(ren) also booked to the respective stateroom. If you really want to be in the same muster station, you might look into changing staterooms - you are locked into fares as of the date you initially booked.
Enjoy your cruise!