I have to say this, so bear with me. I don't honestly think the dining plan is always a good financial move. If you are paying rack rate for the hotel, then you really aren't saving anything.
I personally believe that the most cost effective AND most comfortable WDW stay can be to rent an offsite villa along with transport. We are DVC members so that is part of the reason I say that. Granted, DVC is on property, but if we didn't have that, we'd be going for an offiste villa (which we did for our first visit; not a villa proper but a full guest suite (living room, kitchen, bedroom)).
If you want to stay on property, then definitely look into the value resorts as they are very nice. You can ask for rooms that are connecting too, if that might help with the kids and the grandparents.