Four Value resort rooms would be the absolute cheapeast way ($328+tax per night in Value season, going up from there) but not terribly comfortable. Three rooms at POR would also work ($447+tax in Value), provided you have at least three kids who could sleep on trundle beds---and in my opinion, even less attractive than four Value rooms.
Three Family Suites ($552+tax) or three Ft. Wilderness cabins ($795+tax) would be more comfortable than either of those options. Two 2BR
DVC Villas would be okay too, but maybe a little tight (about $622/night in the least expensive times as a point rental).
The main advantage to being onsite for a large party is the transportation flexibility---if people want to split up, or some want to head back early, etc. it's pretty easy. Offsite, you have to plan around the availability of your cars and/or take cabs.
However I think it is worth at least considering a look at one of the many privately owned vacation homes offsite. For example, you could get a single 6BR home in Windsor Hills that could accomodate your family, with full kitchen and a private pool for about $250 a night, plus or minus.