Through redBus you can buy bus tickets for any minor, but you should bear in mind that when boarding you will be asked for a series of documents. One of them is the child's ID, the other is a letter of authorization from the parents that must be legalized by a notary. Remember that minors must travel with both parents or with the permission of the absent parent during the trip, otherwise the minor can be prohibited from entering the bus and call the authorities in charge, which may lead to a legal problem.