Parental Controls

Take these measures to prevent your kids from stumbling upon adult sites :

THE MOST IMPORTANT THING TO DO

Go the the settings of your device and search for "parental control". This is by far the most efficient and most important measure to take to prevent your kids from accessing adult content. Those tools are widely available and free.

Additionally you can be interested in the other measures below.

ACTIVATE SAFE MODE IN SEARCH ENGINES

Will never appear in the search results with these filters.

Google :
https://support.google.com/websearch/answer/510

Bing :
https://www.bing.com/account

Yahoo :
https://help.yahoo.com/kb/SLN2247.html

And other search engines that you use.

You can also use https://www.kiddle.co/, a safe visual search engine for kids.

WINDOWS 10 FAMILY FEATURES

Once your child has been added to your family with a Microsoft account, you'll be able to set up appropriate website and app and game restrictions for their age, set PC time limits, and review their recent activity. Their settings will apply to any Windows 10 device that they sign in to.

https://account.microsoft.com/family

CONTROL THE APPS IN YOUR TABLETS AND PHONES

Both Google Play store and Apple app stores are already kind of kids-friendly as they do not allow any porn-related app. But you might want to control app installs anyway :

https://www.techradar.com/news/mobile-computing/tablets/how-to-make-android-child-friendly-1045708

REQUEST A FILTER FROM YOUR ISP

Your Internet Service Provider likely has solutions to filter your internet connection at their level. Contact them to find out.

Note that this will block even for you.