The Stay/Play/Dine offer is valid for your dates. So I would price out a trip with the offer, then do another with room and tickets with the 25% room discount that is offered. If you can feed your families for less than the difference, go with the room discount. If not, go with the SPD offer.
One more thing just in case you don't know, a Disney child is only those kiddos aged 3-9. Under 3, they don't get any kind of dining plan, 10 and over are considered adults for tickets and dining. And kids must order from the kids' menu if there is one if you're on the plan, so be sure to check out the kids' menus when you do the math.