In a case like this, your parents would have to return to their home country to obtain their immigrant visa unless they’re 245(i) eligible. Section 245(i) of the immigration law allows certain people, normally ineligible, to adjust their status to permanent residence without leaving the United States, under certain conditions. Generally, people who entered the U.S. without being inspected by an immigration officer, who are or were unlawfully employed in the U.S., or who failed to maintain lawful status in the U.S., are barred from adjusting their status in the U.S. However, there are some exceptions to the last two bars for immediate relatives of U.S. citizens.
Most of the parents in question have been in the United States illegally for one year or more, so as soon as they leave the United States to obtain their immigrant visa, they will be barred from returning to the United States for ten years. Statistics show that there are more than 11 million people in the United States who are currently out of status. Most of them illegally crossed borders and some overstayed their non-immigrant visas. If you would like to apply for family member citizenship, consult with an experienced immigration attorney. An attorney can assist with filing applications, gathering evidence and making the best case for permanent residency or citizenship.