Multiple Contact Registration for events
This situation is why there's the ability to pick a specific dedupe rule for your event's online registration.  The information you get in certain cases is quite different than online donations or other events.

First, unless you are using a dedupe rule where email alone is not enough points to match, you should uncheck the Same email address? box.  The default unsupervised rule says individuals with matching email addresses are the same person, so that's probably causing your problem.

Second, you should consider whether you expect any children to have real, independent email addresses (and whether you'd want those addresses anyway).  If not, make a profile lacking the email address field and use it for additional attendees.

Finally, you'll want a strategy for dealing with duplicates.  If you are just collecting first and last names for additional registrants, you don't have much to go by in matching them to contacts.  (There was another Andrew Hunt at my high school.)  Instead of trying to match blindly at the point of registration, it is probably more efficient to let the form create duplicates and then manage the duplicates later.  Here's how:


Have your profile for additional attendees add contacts to a group for you to review later
You can create a General dedupe rule that matches on first and last name.  It won't be good to use on a front-facing form, but you can use it to find duplicates later.
From the Find and Merge Duplicate Contacts screen, use that rule from step 2 to find duplicates, selecting the group from step 1.  You'll get a few false positives, but you can mark them as "not a duplicate".  Those pairs won't be suggested ever again.
Repeat step 3 every few weeks while your events are getting frequent registrations, and do it at least every couple of months through the rest of the year.