​@AiliMaimaiti 

It is not possible to give you a global answer. It depends of the country, the type of the train, if your child needs a ticket or not, if the reservation is mandatory or not.

If reservations on the train are mandatory, the answer is yes, everyone who is 4 years or older need a reservation and an Interrail/Eurail pass. For night trains the age limit usually is 6 years. 

If reservations are optional, you can make a reservation to ensure that the child gets a seat, in the same way as make reservations for the rest of you.

Children younger than 4 need an own Interrail/Eurail pass is you want to be able to make reservations for them