Call the credit card companies immediately. Explaining the situation may at least stop the monthly service and interest fees. They may even be able to work out some kind of sensible repayment.
It would also be wise to consult a good CPA for assistance in getting their finances in order. He may even be able to help with dealing the the cc companies.
Bring the facts to a good accountant/lawyer. As far as I know your parents are the only responsible parties. But is the debt may be negotiable. Credit card debt is usually taken care of by card company/bank by putting lien on property or against any estate. Good thing is you found out now.
It would also be wise to consult a good CPA for assistance in getting their finances in order. He may even be able to help with dealing the the cc companies.
Good luck!