Youth over 12 need to have a youth pass. They can travel independently.

Children in the age 4-11 can travel on a free pass together with a parent.

Children under 4 travel for free but also need a free pass if you want to book seat reservations for them.

For the children who have turned 12 on the first travel day you need to buy youth passes.

For the children who have turned 4, but not 12, on the first travel day, you need to get children's passes, which are free of charge, when you buy the adult passes. 

If you want to have the possibility to make seat reservations for the child who hasn't turned 4 on the first travel day you need to get a free children's pass for that child too. Otherwise children under 4 travel without passes when they share a seat with someone else.