One of the parents can stay on child-care leave up until the child reaches the age of three

There is a lot of mandatory rules of law in the employment legislation due to protection of weaker party, i.e. for protection of employees' rights. These rules are typically written so that they provide minimum requirements that must be met, but allow better conditions for an employee to be agreed upon in collective agreements and/or locally.

Voluntary work is typically considered as working relationship so generally the provisions of employment legislation apply.

Employment Contract Act is the only law governing employment relationship and terms of employment relationship in Finland. So all the provisions for the employment relationship can be found from it.