For the family of five as it has been said the most economical resort would be the Port Oleans Riverside Trundle room. basically the trundle is 54 inches long, and is designed for a child under 9 yr. old.
I did find a picture of the trundle rooms to give you better idea
http://www.mousetyme.com/DixieLanding.html
the Fort Wilderness Cabins are the most economical in the home away from home category. and with some of the discount codes that come out are very reasonable plus they have a full kitchen.
the next most economical rates for the home away from homes right now are the Villas at Old Key West, and the new Saratoga Springs
Room Type: Two Bedroom Villa
Villas offer views of water, woodlands, or fairways. The two-bedroom, two-bath Villa has a fully-equipped kitchen, dining area, full-size washer and dryer, and private patio. The master bedroom suite includes a king-size bed and whirlpool tub. The second bedroom area has two queen-size beds. The living area has a queen-size sleeper sofa.