If the browser has to do the same thing for adults and children (to avoid detection) then it’s going to be hard to build a system that does different things for adults versus children.
Another way to look at it: a predator could make a website specifically for children and advertise it as such, while covertly doing something bad. How is that different? If they control the website, they can do what they want.
If you’re concerned about that then I think you need a whitelist of known good websites and you need to vet them by browsing it with the child lock turned on. Even then, it wouldn’t be hard to look harmless at first.