Add child badges to MAGStock

[kitsuta]

MAGStock wants to have the same child badge policy as MAGPrime:

How much are badges for children? Do they need to be registered?
Each attendee, including children, at MAGFest will need to be registered and have a badge. All children 6 through 12 get a half-price discount off the current badge price. Children 5 and under are free. Make sure to select the Child registration type to get the discount. If your child is five or under the system will mark the badge as free prior to the order being finalized.