Youchien (幼稚園) or Japanese Kindergarten is a form of preschool for children ages 3-6.  There are both public and private kindergartens. They're all supervised by the Japanese Ministry of Education, but are not a part of the official education system.

In addition to kindergarten there are also hoikuen or Japanese Nurseries/Day-cares for children ages 0-6.  These are supervised by the Ministry of Labor. Unlike the kindergartens they're less focused on educational activities.
園児 保育園