The murphy won't work for your family if your DD is the smallest. The bed is the size of a long toddler bed. It's only about 31" wide (a twin is 38-39") and it is only 5' 5" long so she is already 3 inches taller. I can't imagine how uncomfortable it would be.
Now if you are set on moderate hotels you can book POR or CBR capacity wise then bring a twin blowup mattress. That would be more comfortable.
You also have the option of the FW Cabins, ASMu Suite, AoA Suite or 2 Value rooms before jumping up to the next price range.
When you calculate your costs, if your youngest is 19 that means you have 3 extra adults which adds $45 per night to your costs at POR and CBR. With 2 values only one room would have an extra $10 per night and with the Cabins & Suites there is no extra adult charge. This might bring the daily rates closer together.