Wow, I know you must feel like your home was totally violated. Ugh! Very upsetting.
Can you unzip the cushion cover and take it off or not? We always put kitty litter on urine (like when a kiddo wet the bed) but not sure what to do with it when it's already dry.
I'd be inclined to notify the parents of the kids you know were in your home while you were away, too but maybe that's not a good idea.