Pretty sure you can add or subtract days or upgrade, but not completely sure.
As for using the room codes, you can't do that. The codes that are coming out are applicable to room only reservations. The package is already giving you a discount... kids get free tickets to the parks. If you want to use a room only discount code, you could reserve a room and buy the tickets separately, but then you'd need to pay for tickets for all of your children. If the room only codes are good enough, you may come out even.