Here is a tip:
If you get a B1/B2 visit visa, you usually get a stay either for 3 or 6 months, regardless of your total visa wait time. You can check the stay on form I-94 (departure record white slip that is attached to the passport once you enter the USA)
Try not to stay for the max of 3 or 6 months. Stay for a month or at max, two months and then go back
Once, you do that you are in good books and your chances of getting another visa are 80% more than refusal.
Since, your parents overstayed and they have got one rejection and another negative point is that they are in 70s, means no job in Pakistan or they've almost spent a good life and now in relaxed part of their life.
In order to get a visa, try to impress the councelor that you have strong ties and roots to Pakistan and you will come back by showing either a business or a job (with a job leave for 1 month max).
Now, that you're facing this problem that they can't come back. My suggesstion is to get a good immigration lawyer and if you really desperate, try to make some medical reason that they HAVE to come to USA.
Tell your parents to apply again and tell the councelor that they were going to USA because of two reasons
1. To see their grand children
2. To have their medical checkup (heart or something else)
Make doctor's apointment beforehand (it will cost you nothing to make an appointment and get a written appintment letter and send them to your parents before they appear in interview)
Thats the best I could think
Best of luck