I use this:

Timekpr-NExT (It's stylized as this way) Here's a decent write up of it:

https://itsfoss.com/timekpr-next/

And the source I think: https://mjasnik.gitlab.io/timekpr-next/

Here's our household need for it and I think most people will not like it, but it's what works for us. I have a special needs adult step daughter which has a TBI from a major traumatic auto accident at the age of 2. For most people who see her, she passes as high functioning but that is on the outside. As a result of the accident and brain injury at age 2, In real life, she has problems with the concept of time and time management. She also lacks the executive functioning that most adults have such as the correct decisions in life to make, just to name a few. Having this on her system (Arch Linux) allows us to at least limit the screen time which is what we were wanting. As for filtering NSFW stuff. She's extremely turned off by the thought of people being intimate so, we are pretty comfortable with unfiltered internet. (I also run a DNS server in which if needed can filter traffic).

Another person mentioned using SELinux - this reminded me of using OpenSuSE - that distro is very tuned toward adminstrative access for even basic things such as modifying the network (Well..at least basic for me LOL ) . I think of it is as an ideal OS for small organizations with a single IT person on staff.