Many parent control apps are more like Spyware and are often also exactly that, Spyware which spy on your Kids. Or Google Family Link is also a great example which you can’t remove even if you want too (as parent). I already setup a non-child-friendly blocking DNS and set the Age Limit in the Google Play Store and did all the Privacy related stuff, but I’m still thinking how can I make a Android phone more secure for children? The threads I’m talking about are non-child-friendly content, malware, scams, etc.
Anything other than whitelist-type parental controls will likely be insufficient to block absolutely everything you don’t want them to have access to (or want to have access to them, for that matter).
Honestly, the best way to do so would be no internet access without supervision, which is usually not really a viable option in any reasonable real-world scenario.
The second best way I can think of, albeit a slightly technical solution, is to setup a VPN server at home using a raspberry pi or a similar hosting solution and have the phone connect through that. That way you can control internet access in any way you’d like and even block nasty tracking attempts from apps if you so wish. Most android phones have the option to prevent internet access entirely if not connected to the VPN (this will prevent internet access from any WiFi and mobile data networks). In which case they can still just call or text in emergency situations. The only thing left to do is locking down the phone’s settings, and the rest can be dynamically managed from your network.
All of this does require some basic networking knowledge, but it’s actually surprisingly easy to setup.