This article concerns how to keep little ones from stumbling into inappropriate content on the net via an Android device. If this isn't what you need, you can stop reading now.
Unfortunately, Android has no "master switch" that enables filtering. But there are some things you can do.
I will also say that no filtering is ever perfect.
1. Youtube: Enable "Restricted mode".
How: On the individual device, go to Youtube.com. Go to the bottom where it says "Restricted mode: Off". Click and enable Restricted Mode.
Limits: Only filters Youtube content.
Notes: This seems to work by browser, so you have to do this on the device of interest. Going to Youtube on a PC when you want to restrict a tablet doesn't work.
2. Google Play: In the Play app, go to Settings/Parental controls. You can set a PIN to prevent kiddies from setting it back.
Limits: Only filters Play app content. Play access via browser is unaffected.
3. Parental control app. Several are available on the Play Store.
Limits: None seem to be a clear winner. Most require subscriptions.
4. Use OpenDNS. Methods vary. Basic filtering is free.
Limits: Doesn't work with some VPN services.