Your mobile device and computer have built-in features to make your children's internet use safer. Parental controls are enabled as follows:
Android: Settings > Users > Add user or profile > Restricted profile.
iPhone: Settings > General > Restrictions > enable Restrictions.
Mac OSX: Apple menu > System preferences > Parental controls.
Windows 10: Start > Settings > Update & Security > Windows Defender > Windows Defender security centre > Family options.
Monitoring tools also exist (software for parental controls such as Net Nanny, Screen Time or Norton Family) which help you to monitor (or restrict) the time your children spend online and which sites they visit. These tools allow you to restrict access to certain sites or content. Bear in mind they are not a miracle cure for parents, the most important thing is to explain to your children how and why you use these tools, so that it doesn't feel as if you don't trust them or worse still, are violating their privacy.