When a child was born, you should report the fact to municipal offices.
Foreigners who reside in Japan also report the birth in accordance with the provisions of Family Register Law if the child was born in Japan.
In addition to the report of birth, the new born child needs foreigner registration and acquisition of qualification for sojourning. Please do these reports along with the report of birth.
Please report the case of child birth to the home government, too. Please ask embassy or consulate about the procedures.