User registration without email address OR registration with pseudo-email address
I think you need an email address, but I could be wrong about this.

You can definitely provide a fake email address though.

For example, you can use hook_form_alter() to modify the registration form:

function MYMODULE_form_user_register_form_alter(&amp;$form, FormStateInterface &amp;$form_state, $form_id) {
  // Hide the mail form
  $form['account']['mail']['#access'] = FALSE;
  // Set the email address to minor+UID@mydomain.com
  // You need to write this code yourself...


Make sure you set the email address to include the UID or something similar that will ensure the email is unique; Drupal has to have a unique email for each account.

Then, use hook_mail_alter to check the user's role and, if the user is a minor, query the parent's email address and send the parent an email instead.