To set parental controls in web, Go to youtube website in your browser. On Top Right Corner, click on your image/name representing your account. In the end of drop down Menu, click on "Restricted Mode: Off"

Youtube Menu

Turn the "Restricted Mode" to ON by pressing the toggle.

Youtube Restricted Mode

To prevent others from changing this setting, you can click on "Lock Restricted Mode on browser".

Youtube Restricted Mode

To enable that, youtube will ask you  to sign in again with your google credentials.

That's it!! You have enabled Restricted Mode in youtube and this avoids mature content . However, this mode is not 100% safe. There is a different app called "youtube kids" with 100% kid safety.