Children up to the age of 15 travel free on Sundays and public holidays and during the school holidays in Vienna (this includes the week before Easter, July 6th through August 31st, and Dec 23rd through Jan 6th). An ID card may be helpful to prove a child's age.
暑假去,15岁以下,免费。
http://homepage.univie.ac.at/horst.prillinger/ubahn/english/fares.html