Children need passports of their own
As of 26 June 2012, children must have passports in order to travel. Children can no longer travel with their name on their parents' passports.
In August 2006, Finland granted passports where children could be included in addition to parents. Even if your passport says it is valid for ten years, it cannot be used as the passport of your child/children as of June 25.
However, the parents, i.e. the primary passport holders can continue using their
existing ten-year passports as usual until the expiry date.
Source: Police administration bulletin, 15 June 2012