This is not something that can be done effectively at the network level. It may be more effective at the operating system level using parental controls like Apple's "Screen Time" and Microsoft's "Family Safety" features.
Thanks for reaching out.
I appreciate the assistance, @MichaelP.
@balthesar, we don't have an option to block access to specific URLs or keywords. Google Wifi and Google Nest Wifi devices use SafeSearch technology to block millions of adult websites; however, it currently doesn't block sites with violent and illegal content. I highly recommend sending feedback to our product engineers to add a specific site blocking feature to the Wifi devices.
Feel free to fill me in if you have additional concerns; otherwise, I'll be locking this thread in the next 24 hours.