Can my parents leave and reenter the US on their current B2 visa while their I130 (filed while they were in the US) is pending?

Assuming that your parents have maintained their legal statuses while in the US and have the proper nonimmigrant intent to only visit and go back to the home country, they could still be allowed to reenter the country on their visitor visas even though they already have I-130 petitions filed on their behalf. They must of course tell the truth to the immigration officer at the port of entry if asked whether they are being sponsored for green cards. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.