Some Home Broadband Routers offer setting up restrictions too.
Some Apps have preference Settings.
There are in some cases alternative apps for kids to use. Like not using YouTube but using a children’s version.
When my children were younger I found sitting down with them and explaining the wonders and dangers of what the internet can bring helped too.
After that I had to trust them and if they betrayed that trust then I would have removed the device.
The point I support is to monitor children and not directly interfere with their activities. I recommend that you use the Android Parental Control app, which is more targeted for your child's monitoring. I've been using KidsGuard Pro for a while, it's better than the app I've used before, and if I'm happy with it, it will be my last choice.