You’ll definitely need a plugin for that. It’ll need a custom field for the parent address. Also, do you think you want to send the email when they log in, or when they visit the site? Users typically need to log in only once unless they make a point to log out. If someone told you that the email needs to be sent when kids log in, you should not tell them what I just told you. 
Since you are the first person to ask for this, it sounds like bad legal advice to me. But there is a long history of schools following bogus advice.