I’m so sorry that you — and millions of parents — are caught in this situation. YOU are responsible for the payments! And PLUS loans are the most expensive loans out there and carry the highest rates. And they typically cannot be refinanced.
I’ve said that for years!
Now, your only option might be refinancing your mortgage and taking out some equity to pay down as much as possible on the loans. The interest on that part of your borrowings wont be deductible, but it will be lower by far than the PLUS loan interest rate.