There are numerous app and websites that are geared towards monitoring your child on the Internet, across all social media platforms, and on text messaging. To get started visit Common Sense Media's Ultimate Guide to Parental Controls.
Google Family Link: Lets parents monitor and manage kids' Android devices. For a guide to better understanding Family Link check out https://www.commonsensemedia.org/blog/how-to-use-googles-family-link-app
Apple Parental Controls and Child iTunes and Apple Screen Time are built into the Apple IOS and help to limit and monitor child usage on ios devices. For a more information on Apple Screen Time check out https://www.commonsensemedia.org/blog/how-to-use-apples-screen-time-feature