I'm not sure if you can add the dinning plan to the room only discount. I know if you want the free dinning you need to pay rack rate and add on park tickets for at least 2 days.
Right now I'm thinking we'll try for a room only discount and get our park tickets through Undercover Tourist.
Yep, I ended up making one of our ressies this way. So, if both are offered around the time you want to go, you can just see what makes the more sense for your family. Free dining or room discount pay for DP if you want it. If you're staying Deluxe, room discount way and add DP will probably always be cheaper.