This is a special fare that applies to children between the ages of 2 through 11 on the date of departure. Therefore, a booking made before a child's 12th birthday for departure on or after their 12th birthday will no longer be eligible for the 'child fare'. In most cases, children are usually charged a percentage of the applicable adult fare.
Learn more about the child fare and the baggage allowance through this link
Contact your local Qatar Airways office for further inquiries. Contact details can be found in the following link https://www.qatarairways.com/en/worldwide-offices.html