All prices quoted by Disney always assume double occupancy. So, if they are quoting an adult price at $400 (let's say), they are assuming the adult will be sharing that room with another adult for the same $400. So, in your example, that 5th person would be a higher charge, since they wouldn't be sharing the room with anyone, and you'd need two rooms still.
Edited to add: After re-reading your post I'd like to clarify a little more. If you're traveling with two adults and three children, the childrens price is almost always just park tickets, usually no room component is added in their pirce. The adult price is always quoted as double occupancy (which means that they divide the room fee by 2 and add that to the per person quote for adults, but assume 2 adults will be in the room) and has a room component and a ticket fee. To assume what you're trying to do, you'd have to price your children out as adults to get 2 separate rooms, since that includes the room component as well as the ticket component. You can not pay for 2 adults and 3 children and be placed in two separate rooms, since the children's price doesn't include a room component. You'd still be responsible for that additional per night charge for the additional room. I hope that makes what I was trying to say a little clearer.