I agree so much with this. We all know, and often complain, that 'guests don't read' so it's highly likely that they didn't know that they were doing anything wrong. In this case too, it was their own daughter who was in the rental and not some random hookup they'd picked up off the street.
Every host is different but I would have mentioned it to the guests when I first saw the daughter - something like 'oh, this is your daughter, good to meet you. You know, having extra people round is against our house rules but I'll turn a blind eye in this case and of course, she won't be sleeping here so there's no extra charge'.
That just me though and I've done it several times with the result that everyone has been happy and the guests have been grateful that 'breaking the rules' hasn't had a negative impact on their stay. In my case, an extra person for a few hours doesn't cost me anything and only a minute amount for one extra shower.
Most times I've mentioned it in the private feedback ('other hosts might not be as lenient!') and dropped them a star for house rules.