Question about duplicate matching rule for event registrations
Even though many children/youth these days have their own email address - parents will often use their own for registration/event purposes - b/c they aniticipate (correctly so) that's where receipts/reminders will go. 

For such registrations you need to use/create a rule like first name + email address [as last name is often also the same within a household]. 

I prefer Drupal - webform-civicrm for this as it allows me to set up one specific matching rule for the Primary Contact [default unsupervised] - and then I can set up different matching rules for the other (children) Contacts on the form.

See https://civicrm.org/blog/sarahgladstone/recipe-for-handling-youth-registration-with-civicrm for a nice write up/example by Sarah on how this works.