I would like to know how to change a child's age in iCloud

Apple ID - Update the birth date associated with

If it's a child account (generally under-13) only iTunes Support can potentially change it : http://www.apple.com/support/itunes/ww (click on your country's flag, the country where you live, and which is therefore the country on the account ; click on Contact Support towards the top right ; click on Get Help under 'Talk To Us’)