In the case of children under 12 travelling in the company of an adult, no ID is required when buying tickets.
For children travelling alone, a responsible adult must submit an Authorisation Form for non-accompanied minors. In this case, it is not necessary to show the minor's ID document during the purchase process but, on boarding, the accompanying adult must show the driver some form of identification for the child (ID card, passport, Family Record Book, etc.).
Remember, children under 16 may not travel alone on international routes and may only board the bus in the company of a responsible adult.