I had a coop very similar to this, except it was longer than wide. And it was closer to 3 feet tall, maybe 4 feet at the peak. It was basically a nightmare. Very difficult to catch birds, very difficult to clean, and often times they would not use the nest box, but rather that far corner where it was nearly impossible for me to reach.
The real reason you want them to roost, is so that the poop falls away from the bird during the night. They poop an amazing amount at night, and if they are squatting in it, it makes for dirty feathers, dirty eggs, and even skin irritation.
It sounds like you are handy, on one side, enclose the bottom walls, and cut the floor out. They can roost on that side, the other side can be used for nests.
If your daughter continues to enjoy this hobby, well this would make a perfect brooder or second coop, and yes, it is a bit scary how many of us have that second coop.
Mrs K