I would rather recommend installing some parental control system. You see, apps can solve few problems limiting only a couple of sources of harmful information. Believe me. My teenagers taught me to block everything possible to protect them. A year ago I was thinking just like you but read a lot of reviews and found the system called mspy. It helps me supervise my kid's Internet activity and view the list of all visited websites and block the unwanted ones if needed. Moreover, I can keep track of every photo and video saved on their phones. Every time they record a video or take a photo with their device, it is immediately be uploaded to my mspy account. Highly recommended. Cheers.