Adding my child’s iPhone to my list of phone under Find my iPhone

What you do is set up Family Sharing and give your child their own Apple ID, then share location.

see these links below...

Family Sharing - Apple

Family Sharing and Apple ID for your child - Apple Support

Share your location with your family - Apple Support

Use Find My Friends: About Find My Friends - Apple Support

and also see the "Share Location" section of: Family Sharing - Apple Support

You're welcome!