It is not possible to give a complete and definite answer to this question because grandparents' rights are a matter of state law and some of the details of your particular situation might affect the answer, no matter what state you live in. I can give you some general information, though, and a few links to help you.
Some states do not allow grandparents to sue for custody or visitation unless the child's parents are divorced or separated or one parent has died. I do not know if that is your situation, or if your son and daughter-in-law are together. There are about 20 states that make divorce, separation, or death a requirement for you to have visitation or custody rights. Some states do not have this requirement, but can still make it difficult for grandparents. Most of the Supreme Court case law in recent years has limited grandparents' rights.
It is a very sad situation when a parent will not allow children access to their grandparents, but sometimes people can work things out without going to court. If you cannot work things out without going to court, you will need to see an attorney in your state. Sometimes county bar associations have a service that allows people to see an attorney for a preliminary consultation at a very low cost, and sometimes there is a local legal services office that can help for free. What you do not want to do is arrange to see your grandchildren without their parents knowing because that would be wrong and make it difficult for you to get any rights at all if you do have to go to court.
I wish you the best of luck with this situation.