I have setup a rpi wifi hotspot which I can switch on and off using NodeRed. I use it to control my kids internet access time. Trouble is, I have no idea what they are looking at and they seem to be getting more and more secretive. I wonder if I can see the URLs they are accessing. My goal would be a NodeRed page showing a history of URLs which I can see and also click on (maybe with client MAC address too). I am guessing all DNS requests would need to be logged to a file. Which system generates this info?
There are several considerations here. The straightforward way is to log what the DNS forwarding (you've probably enabled) in your pi does. Typically, this will be a service such as dnsmasq, depending on how you set up the hotspot functionality of the pi. However, that will only tell you which URLs your children where browsing, and could include a lot of pages, even ones not directly opened (i.e advertisement services from third party locations) - hard to analyze.
And then, more and more websites use HTTPS and eventually secure DNS, where it will be next to impossible to track the content of the sites somebody is browsing. Consider also that, if your children use sites such as youtube (which they are probably generally allowed to), you have no way of knowing whether they were looking for proper learning videos or something adult-y, since all of that is encrypted.
Now there are ways to intercept SSL communication in a private network, but that is very complicated and probably not easily possible without special software.
Bottom line: I'd try to go the manual way and talk to your children that it's important that you know what they're doing and watching on the internet. After all, this is a social problem they have to learn, and trying to solve social problems with technical measures may not solve the original problem and may cause the social problem to rise (as people start to distrust each other).
The above not only applies to parent-children relationships, by the way, but also to companies and employee-relationships. In many countries (in europe at least) network supervision by employers is generally forbidden - any suspicions on dubious or even illegal activities by an employee first need to be addressed in person, and only if that doesn't help, technical measures are allowed.