How can I supervise/monitor the iPhone of my kid? I use an iPhone 8 and  my 12 year-old  daughter had an iPhone 6 just purchased.

I'm worried about Internet sites mainly and would like to monitor text messages of certain friends she has been in trouble with at school. I have spoken to her about this and it was one of the conditions she was allowed her own phone. Can my iPhone monitor her web sites and sms texts ? Can I block both from my phone?  I have seen some apps but was told they could be circumvented.
Parental controls are more so about restriction rather than monitoring. Although your intentions seem honest and reasonable, someone could maliciously use a feature like this to spy on friends or significant others.

Explore Apple's robust iOS Parental Controls instead.

Parental Controls