We may run a computer with linux, if that changes anything. What are people using?
It's very cheap ($1.99/mo) and has an app (macOS/Windows/iOS/Android) that provides filtering/monitoring on the go, even when they aren't at home. Also block ads and malware sites.
The best you can do to protect kids online is to teach them the importance of online anonymity, why ad blocking is essential and how to avoid infecting a computer with malware.
Also, the internet is currently being scapegoated by the media for the teen mental health crisis but this is only happening in the US and other countries also have teens with internet access. Just like school shootings, this problem is being blamed on scapegoats because the real causes of the problem are politically untouchable and want to deflect the blame on anything else to stay that way. The moral panic over teens and social media is really just the "Grand Theft Auto is turning kids into murderers" of this generation.
Beyond that, I will look for the good replies on this post - I am leaning towards "big hammer" blocking and allowing/curating sites/videos. The Internet is a big place.
What is certain sites? X-rated? Political views different than your own? Only allowing religious content?
White listing for total control but high effort and use a black list less control but less effort. You could always not allow usage without a parent present but that's extremely high effort.
I would go with good adblockers and a domain-tool like Pihole or similar. They usually have some decent block lists that will catch 90% of accidental attempts to visit bad places, and blocking ads cuts out 99% of the temptations there anyway.
Put the computer(s) in a public place in the house, make sure they have a decently sized screen so parents walking by can see what's going on.
Make sure the kids know they will never get in major trouble by telling you what is happening, and no trouble at all if someone (online) told them not to tell you - those are big red flags.
Show genuine interest in their activities, and equip them to make their own decisions when using the internet.
My eventual plan is to have a tool that works independently for each devices and home-ish setup that I can keep tinkering while taking care of the home-devices that stays home.
Then setup each kid with their own accounts and add them to the fam sub.
1) this removed the absolutely revolting adverts. and revolting aside, it stops the videos being interrupted. Kids these days self educate with youtube videos (my sone taught himself to code via youtube videos). you dont want adverts interrupting that process. (I live in hope that one day youtube will let premium subscribers turn off "shorts")
2) log into the kids youtube accounts after they have gone to bed every few days and unsubscribe them from the rubbish you know they'll click the sub button for, and then subscribe them to things you'd like them to see more often. They will have no idea you;re guiding their viewing experience.
We have pi-hole too, primarily for blocking ads and call-homes.
It won’t work if your kids install and use another browser on the device (iCab, Firefox, etc.), but I suppose you could lock down app installations on whatever iCloud account is set up on the device (or with Parental Controls).
The subscription is fairly inexpensive and the app’s blacklists are updated regularly. As a bonus, it blocks in-app tracking.
(No affiliation, just a happy user.)
* Not buying them smartphones or tablets or laptops.
* Only allowing them access to a desktop PC (or even a Raspberry Pi) with a large monitor in a public space like the living room. You don't have to watch over their shoulder, but there's only so many things they'll look at while worrying about "being caught". And if they have problems with excessive usage that interferes with their other responsibilities or just the house rules, you can just take the power supply.
For my young, innocent children, my goal was to protect from accidental clicks. Ad-blocking DNS (pihole, NextDNS) is very effective for that.
Then they got a little older and had friends showing them gore and horror things (because they had unlimited access). This was not age-appropriate content and resulted in multiple sleepless nights as they tried to process the images. Not wanting to perpetuate the cycle, I installed Qustodio on their Android devices. I'm satisfied with that.
I recently bought a Synology RT6600ax and love it. It gives the ability to assign different devices to different sets of network rules so I can customize my parenting to my children as they go through different developmental phases. Having this is a good complement to qustodio because you can configure it to shut off internet for all devices in one place. It does take a technical user to use it to it's potential though.
The no cell service will be the most difficult, we gave-in to pressure from grandmother, it was a mistake.
Now let's say, you have your children's devices on their own separate SSID on its own Vlan, with some DNS restrictions, oh wait they can just turn on their personal hotspot on their phone and connect their other devices to it because Screen Time has no way of disabling that feature, and AT&T has no ability to do this on their own (Verizon can do this however).
If they're old enough for a phone, they're old enough to figure out how to get around your blocks. Our 12 year old was informed of a Discord server (at school by other students) specifically for sharing proxy websites that get around website blocking on their devices. And the urls were all inconspicuous, like a website about minecraft skins has a subdomain just for hosting a web proxy.
You can do your best to block, but education around bad actors and what to look out for on the internet is far more valuable here imo.
The short answer is, "No, there is not. There are simply ways to make it harder." However, there's no shortage of companies happy to sell you things they claim will do that - impossible though it is, having been a kid, and having done a lot of network related work in my years.
What you can do is provide an environment in which it's harder (not impossible - simply harder!) to run across stuff unintentionally, and ensure that computing hardware is an environment where it's more difficult to start intentionally bypassing boundaries set up.
A Pihole is a good start, but it's also trivial to bypass unless you lock down your network pretty hard, prevent external DNS servers from being usable (blocking UDP/TCP 53 outbound from everything but the pihole goes a long way here), and manage to block DNS over HTTPS (which is a bloody nightmare), as well as external SSH connections you can SOCKS proxy over. Assuming there's a good endpoint. However, the Pihole will help keep the amount of crap down, and should do a decent job (with proper blocklists) of helping prevent unintentionally following links one doesn't care to follow (think "hosts entries to block certain goat and tub sites," if you were on the internet in the early 2000s).
Any computer a kid is using needs to be in a public space in the house, with decent foot traffic, and you should ask them what they're doing often enough that it's clear you're paying at least some attention.
No cell phones with browsers or mobile devices until they're quite a bit older (mid teens, maybe?). If they need a cellular device, get them a flip phone.
And then beyond that, it's going to be kid specific.
And make sure there are plenty of things to do that don't involve computers in their lives.
The book that just came out, "The Anxious Generation," by Haidt, goes into some detail on the results of our experiments of more or less unlimited internet warfare on the development of kids, and the results are an absolute disaster, especially for tween/teen girls (in terms of mental health, suicide, happiness, human connection... pick your metric, it's been a disaster for them).
And if you think this is over the top, go spend some time on r/Parenting and look for posts by teachers talking about the sort of absolutely absurd stuff they see on phones from kids in grade school and middle school...
This is all based on blacklisting so is far from infallible. You'll still need to take some interest in what your kids are up to.