Legal Permanent Residents can petition their unmarried children who are over 21 years old by filing an I-130 petition for them. These minor children, are however subject to visa quotas in the F-2B category, and must remain unmarried until they actually get Legal Permanent Resident status.
The F-2b category is currently backlogged for all countries, which means that the unmarried children of Legal Permanent Residents are not able to file their I-485 Applications for Adjustment of Status when the I-130 is filed. They will have to wait for their priority date to become current. While waiting for the priority date to become current, these unmarried children must either maintain legal non-immigrant status in the US, or must be outside the US.
Once the priority date becomes current, these unmarried children would file for adjustment of status if they are in the US, or would apply for an immigrant visa at the US Embassy or Consulate if they are outside the US. If they are applying for an immigrant visa at the US Embassy or Consulate, they cannot get married until they have actually been processed for the Green Card at the port of entry in the US.
It is highly advisable that you at least consult with an immigration attorney. Our Los Angeles immigration lawyers are available to advise you.