It's my experience that you can change resorts after a code has expired (depending on availability), but cannot book another room.
If there are only two people booked in the OP's POP room, OP can add parents to the existing room reservation/package, pay the extra adult fee, and then book a separate room (on a room only basis) that the parents actually will occupy.
If the 3rd and 4th people in the existing POP room are children, then the OP should book the parents as the 3rd and 4th in the existing POP room, taking the children off the reservation.
Then, book a separate room (on a room only basis) for the OP's family, which is the room they actually will occupy. That way, at least the adults can all get the
free dining and they can pay for the kids out of pocket. Even with the extra adult fee, getting 4 adults onto free dining is worth more than keeping it 2 adults and 2 kids (assuming the kids are 9 or under or, if over, 10, don't eat a lot).
I very much agree with the PP about being very careful in doing this. I always say straightaway, "Do not change anything yet. I'm just interested in what my options are. Do not touch my current reservation unless I tell you to."