Even children of Japanese couples may not acquire Japanese nationality.

Usually, even if a child of a Japanese couple is born in a foreign country, the child will acquire Japanese nationality.
However, if a child born in a foreign country acquires foreign nationality along with Japanese nationality, he or she must submit a notification of birth and a declaration of intent to retain Japanese nationality (notification of nationality reservation) within three months from the date of birth.
If you do not, you will lose your Japanese nationality retroactively to the time of your birth.
Therefore, if a child without Japanese nationality stays in Japan with a parent who has Japanese nationality, he/she will stay in Japan with the status of residence of "Spouse or Child of Japanese National."
If the child meets the following requirements, he/she can acquire Japanese nationality by notifying the Minister of Justice.
 (1) be under the age of 18 at the time of notification;
 (2) have an address in Japan; (You must be based in Japan.).
In order for a child who does not meet these two requirements to acquire Japanese nationality, it is necessary to apply for naturalization.
Japanese people who plan to give birth overseas in the future are recommended to submit a notification of nationality reservation along with the notification of birth.