Take these measures to prevent your kids from stumbling upon adult sites :
Gejswiat will never appear in the search results with these filters.
And other search engines that you use.
You can also use http://www.kiddle.co/, a safe visual search engine for kids.
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.
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 :
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 Gejswiat even for you.