These are the conditions when you travel with children on ÖBB trains: 

• Children from 0 to 5 years old inclusive: they travel for free when accompanied by an adult but they must be included in the reservation at the time of booking and will be given a free travel ticket. Please keep in mind that children riding free night trains will not have their own bed. If you want to reserve a bed, you must purchase a Child ticket. 
• Children from 6 to 14 years old inclusive: get a 50% discount on their ticket. The discount will automatically be applied when you enter their age. 
• Teenagers from 15 years old: the full price will be charged.