My first thought was that if you didn’t have the camera and everything seemed okay when you returned home then there wouldn’t be a problem. You would never have known. So really you’re just stressed because you have security cameras, like someone who was perfectly happy with a restaurant until they happened to look in the kitchen.
But on the other hand, that’s like saying you got drunk last night and drove home, but you didn’t have any accidents so it’s fine. It’s not fine is it? You wilfully created a situation where it was more likely something could go wrong.
Kids are more likely to break things, especially in homes that are not set up for children. Kids fall over and hurt themselves more often than (sober) adults.
And while most people will take responsibility for their own / their kids’ actions, you do have a legal liability if somebody gets hurt in your home. If you don’t know these people, how do you know they aren’t going to make a claim against you? Unlikely, but more likely than it would be if they weren’t there.
It’s the same with driving badly and carving up your lawn. You can hope alcohol wasn’t involved, but you don’t know what went on. Is this person just not competent? Are they going to reverse over a child as they leave because they’re not really in control of the vehicle? Again, you don’t know.
I generally only sit for hosts that are very relaxed and permissive, and I think they generally accept me because I present as a level-headed, responsible, and self-sufficient person. I am low maintenance, and prefer a quiet life. So I don’t want to deal with people who have lots of rules and requirements, or who worry about every little thing that can go wrong and what the worst possible outcome could be. But I understand where you’re coming from.
The sitters have taken liberties, and they probably never stopped to think about the deeper implications of that. It could be down to inexperience, confidence in their own abilities to deal with anything that might go wrong, or contempt. Or some combination of those factors.
They created a higher level of risk for you, against your explicit instructions. It seems like no harm was done, and I imagine that in most cases no harm will ever be done. But if thousands of sitters routinely bring extra guests into other people’s homes without permission - or when specifically told not to - eventually somebody is going to create a mess that the homeowner didn’t sign up for. Whether it’s red wine on the carpets, broken ornaments, a five-year-old sharing chocolates with a dog, or a poorly-driven car smacking into something, it’s over and above what you agreed to.
I think if I was in your situation, I would try to gauge what the sitters’ thought processes are. If they’re just irresponsible and contemptuous then a bad review is definitely in order. Without consequences, there is no reason for them to modify their behavior and other homeowners need to be warned.
But if they hadn’t really thought about it, you might be doing them a favor by opening their eyes. After all, they’re the responsible party if their guests do any damage to your property or hurt themselves.
I have found that explaining things to people in terms of best practices is usually more effective than telling them they’ve done something wrong. “Sooner or later these things will happen to someone and so we have rules to make sure they don’t happen to us.”
Hope that helps, and no harm is done! It’s New Year’s Eve, are they having a party?! 
