Well, it depends on how detailed you really want the information. Most box store routers will log the sites, but not necessarily the web pages. If this is good enough for you, then they will do the job for you. You may want to look into setting up a 'syslog" server (software) on your PC, so the router (some models) can send all the logging direct to your PC as it happens. This prevents the "kids" from power cycling the router to clear the logs. However, if your PC is not running, you won't get that information.
Almost any router has this weakness, if they power cycle it, the logs disappear. That's because the router does not have a disk drive to record the information permanently.
You could leave a spare PC running all the time, and send the logs to it. Even if they power cycle anything, the logs remain on disk.
Some higher end Linux firewalls (free, "just" have to build them on a spare PC...) have logging build into them, onto their own disk drives. With a web GUI (your browser), you can monitor the system and check any activity. As well, you can manage what sites are allowed or blocked, along with what protocols are allowed or not. In other words, there are more advanced tools to control "the network". However, this usually requires a bit of skill in installing these boxes, something most people don't have. But, if you even got one of these running, you would know everything the kids are doing, and be able to control all network access. It all depends on how "serious" you are about watching the kids... Sometimes it just as easy to install a blank PC, and tell them that it is monitoring everything, rather than actually doing it lol...