How long were you married to the citizen? If it was at least 2 years, and your ex filed the necessary paperwork, then your "conditional" green card would be permanent after 2 years. But from what you're saying, I don't think you have your green card.
The other issue, besides green card, is the issue of taking children in and out of the US without consent of the other parent. For example, if I try to take my children out of the US, I may get stopped at the airport/customs because that could be perceived as kidnapping. My spouse would have to agree to let me take the children out of the country. This is especially the case when the parents are divorced. Is that what's going on?
Or is it that your children who are minors have US citizenship and you are trying to enter the US and you can't get a visa? In an attempt to contact your ex for child support? Even if your children are US citizens, I don't think you can use their citizenship to enter the US. If this is the situation, then I have no idea. Maybe this website can help you: ACA American Citizens Abroad