We have the same issue. I have 7yo triplets
and have dealt with this for the past 3 years. On one of our trips, my parents came with us, so we rotated one child sleeping in their room each night. But for all of the other trips, we have always made a reservation for 4 and just put the 5th person in the room. Disney has never questioned us when we show up at the reservation desk with 3 kids. I know I was bucking the system, but at the time just couldn't afford the additional expense of the larger rooms they required for 5.
If you are booking a package, you will only get the package deal for 4. You will have to buy an additional ticket for the 5 person. Also, if you do dining plan it would only be for 4 and you would have to pay by the additional person.
I would highly recommend staying at the Port Orleans Riverside. You can get a room in the Alligator Bayou section. They are set up with a trundle bed under one of the beds to handle the 5th person. Also the rooms in the Moderate resorts are bigger and just make life a little easier with 3 little ones. We have found having the two sinks that the Moderate resorts have to be a big help as well.
Do what is best for your family. If you need to put 5 in a 4 person room, you can do it. I did a lot of pricing in the last few days as I just booked us for Mardi Gras week because of the Buy 4 Get 3 free deal. I found that booking the 4 person package for just tickets and rooms at a Moderate resort and buying the 5th ticket separate were the same if you booked the package for 5 at the Moderate resorts. Obviously, if you can get the 5 of you in a Value resort you will save money booking the 4 person package and then adding the addtional ticket compared to having to book the 5 person package at a Moderate resort.
Hope this made sense as I know I have rambled on quite a bit and it is getting late.