We have stayed at the cabins with 6 people, not all of them adults. There is a pull down murphy bed in the front living area, but trust me, there is no extra room for a cot or air mattress in the cabins. Remember this--there is only one bathroom for all 6 people, so it might feel a little crowded after a while. For a very small child you could possibly put an air mattress down in the kitchen, but there's just not that much extra space to move around.
The cabins are great, and we love FW, but if you really need extra space for more people, why not look into renting a condo. The other alternative might be to rent a campsite, and take a tent with cots and supplies. Then all of you could still use the cabin for cooking and a central meeting place.
Hope that helps!