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

• Children under the age of 6: do not require a ticket or a seat reservation; if you add a child under the age of 6 to your booking, their name will not appear on your e-ticket. 
• Children between the ages of 6 and 14: travel for free in Germany (when accompanied by their parents or grandparents). They must have a ticket to travel, so include them as a passenger when making your reservation. Usually, they cannot travel for free on cross-border journeys between Germany and France. You must buy them a ticket if traveling between these two countries.