monitoring child's phone

Screen Time will let you monitor your child’s activity including amount of time spent on device (with a pretty good level of detail on where and how the time was spent), set limits, block explicit content, and more. You do need to set up Family Sharing if you haven’t already: Set up Family Sharing - Apple Support

And you should create an Apple ID account specifically for them as described here: Create an Apple ID for your child - Apple Support

Hope this is helpful!

Yes, using a feature called Screen Time. Here’s an article about it.

Use Screen Time on your iPhone, iPad, or iPod touch - Apple Support

Ask them to see their device