It has been over 4 months since I last saw them. She is planning to go abroad in December to get married not on paper but a religious ceremony and wants to take the kids. What can I do or what is the next step please?

First, where are you located? Laws vary by area. Do you have an attorney helping you with the divorce? Why haven't you filed for a temporary visitation schedule?

Generally children need permission from both parents to leave a country.

A wedding can be a religious ceremony but a marriage is a civil action. She can't be legally married until your divorce is final.

But the bottom line here is you need to push the court to get and enforce visitation/custody orders.

Your divorce decree should specify what your visitation rights are. Is your ex planning on moving abroad permanently with the kids? If so you need to get your divorce attorney on this pretty quick.

You need to push for at least a temp custody agreement or an emergancy hearing, to resolve custody, visits and child support.

In it, issues of leaving the country, or moving out of state can be addressed also

it has been over 4 months since i last saw them. she is planning to go abroad in december to get married not on paper but a religious ceremony and wants to take the kids. what can i do or what is the next step please?

You need to be more precise with you language.

You use the terms "divorce" and "partner". The two don't usually go together. The correct term for someone with whom you are married is "husband", "wife" (depending on that person's sex), or "spouse". Are you presently married to this "partner"?

So she is planning on a religious ceremony without actually having a marriage license? Depending on where she does it, the minister who performs such a marriage may be guilty of a crime by doing so.

And of course, if you are indeed married to her, if she intends to actually get marred (as opposed to, what, a make-blelieve wedding?), she would be guilty of bigamy, a crime in many countries. And/or such a wedding would be void.

As far as taking your children with her is concerned, as others have said, that may be prohibited by the law of wherever you are.