Japan allows dual-citizenship only until 20 years old. At that time, the person must decide on only one. If the other country allows dual-citizenship regardless of age, many people tell the Japanese government that they are choosing to remain Japanese but then they just "forget" to renounce their other citizenship. The Japanese gov is well aware of this but kind of ignores it. So in that kind of "don't ask, don't tell" way, yes, Japan allows dual-citizenship.
That's interesting, I guess one day your children could have some benefit having dual passport since both are golden passports too 😁 especially Japan.