@Ryan2147 There are tons of posts on this forum that make it clear that there are many guests like this- the instant they feel they are being criticized or blamed for anything, no matter how tactful your approach, they counter with some made up b.s. about something you know perfectly well isn't true.
It also amazes me that there seem to be so many people who refuse to take responsibity for anything. I'm not accustomed to hanging out with people like that and can't imagine damaging someone else's stuff and not bothering to mention it, apologize, and offer some compensation.
Now of course it is possible that the parents didn't realize their boys had managed to break the sofa. I raised three daughters who weren't at all destructive, but when they were teenagers, all their friends used to hang at our house and my experience was that teenage boys are really hard on things. It's like they don't quite realize how large their body has gotten and how strong they are and think they can still throw their bodies around as if they were little kids.
These people are obviously not going to pay for the damage and as you say, will no doubt repeat their dirty dishes lies in their review. There's no point in you failing to leave an honest review, but if you don't get notice that they have left a review, wait until day 13 and 3/4 to leave yours, so they don't have time to leave one themselves. If they do submit a review before that, no point in waiting to submit yours.
If it's not a really expensive thing to fix, maybe you'll just have to chalk it up to the cost if doing business if Airbnb makes it difficult, rather than spend hours pursuing compensation.