Hi Jeremy!
My first reaction to your question was to recommend the newest resort on Walt Disney World property,
Disney's Art of Animation Resort. First, it offers family suites that can sleep more than four, and second, it has, what I consider to be some of the best theming of any resort on property. There are Little Mermaid rooms, but not in the "suite" section. A great choice for you could be a Finding Nemo family suite.
Other resorts also have limited rooms with a fifth bed (usually a trundle or sleeper sofa), and there are rooms at the Vacation Club resorts that offer accommodations for a family of your size, as well. Here is a link to a great resource for
finding rooms for families of five or more at Walt Disney World. It outlines the resorts that you can choose from.
You can also always call a Disney reservation specialist at 407-939-1936. They can help find the best resort for your needs.
I hope this helps! Please let me know if you have any other questions.