I'm a retired USAF, USA citizen. Ex-wife, Japanese citizen. Japan (JP) child custody laws prevents joint custody in JP divorce. I, being a foreigner was a big strike against me. JP law/culture awards mothers full legal/physical custody. Father has none, especially as a foreigner. JP court wouldn't put joint custody in the decree. I asked if they could state that I have unlimited visitation rights. But what ended up being in the decree is "applicant accepts visitation with the child to the respondent". There are times when we have disagreements and she would say she's his mother and can do anything she wants because Japan law awarded her full custody. All I want is 50% legal and physical custody in the U.S. I just want my son and I to be protected. I have rights as a father but not in JP.
You most likely need an attorney who knows and is licensed to practice Japanese law.
From the facts you have supplied, it does not appear as though returning to a Japanese court to revise your decree would be helpful to you. You should certainly check with local counsel in Japan to get their opinion on that. Failing same, it seems like the question you are really asking, is whether a US family court can modify the custody or visitation provisions of your Japanese divorce decree. This is governed by the Hague Convention on the the Civil Aspects of International Child Abduction. Don't be thrown off by the title. The Hague Convention essentially provides for which state/country has jurisdiction to make decisions affecting custody and visitation in international child custody cases. It boils down to the notion that the state/country with jurisdiction will be the state of your child's "habitual residence." Do your children live primarily in the USA/Hawaii, or do they live primarily in Japan? This will be the primary guide as to whether a US court can exercise jurisdiction to amend the custody/visitation provisions in your Japanese divorce decree. There is an exception in cases where return of the children to their country of habitual residence would result in a "grave risk" of harm to the child, in which case the non-resident state/country could exercise jurisdiction.