Broadly speaking, if you overstay your visa and do not have a prior application for an immigrant visa filed prior to April 30, 2001, you will not be able to get lawful status in the U.S. There is the possibility of an I-601A waiver based on extreme hardship to a U.S. citizen relative, but no facts have been shared that would support such a petition.
The parents should discuss their situation with an immigration lawyer to try to determine if any options are available to them.

