Has your 6 y/o ever been around a chicken? I think, with a little bribing and allowing the kiddo to pick and name their own chicken or two for the flock and promises of gathering eggs you just might get that coop back!
I agree that you've got to enjoy kids while you can, because they grow up fast. And your coop does look like the perfect fort for a 6 year old!
Can you post an update. Is it still a fort, or have you been allowed to convert it to a coop yet?