At Disney, you need a room key for everyone in the room because you need to present it at Extra Magic Hours and for other, can't think of any, small benefits. I think the Extra magic Hours is the main thing. You can put non charging privledges on the childrens keys. Oh, they will also put your park ticket on your room key in certain cases.
Disney most certainly allows this. You can get the room key, get magical express, and the meal plan for that 5th guest. They know way in advance that a 5 guest party is checking into a 1 bedroom unit. If you are a member of DVC, the official policy has been 4 in a 1bedroom unit plus an infant. The unofficial policy has always been, you can put 5 in, but they will not provide extra bedding or towels. There is much debate on whethor this is a good thing or a bag thing. I won't get into that here. You can always find threads debating the issue. Now this policy is for the older resorts and units.
The newer resorts have the extra bed and the official policy is that they will sleep 5 or in the case of a 2 br, 9.
Exchange confirmations will only show that the max is 4.
When we do the 5 in a 1 br room it is usually, myself and my 3 children plus my husband's niece who is a preteen. So there is usually plenty of room, for 1 adult and 4 children. I can see this changing in the future now that my older two are adults, almost.