DISCLAIMER (Once again): I am not a therapist. I am not a lawyer, attorney or barrister (I am a notary). As accomplished and unique my skill-set, I am not those occupations. I am a mother who at one time was my younger children's grandmother.
Charney Herst wrote a very good book, For Mothers Of Difficult Daughters. I recommend anybody with difficult adult children, no matter the gender, get it, read it, and then reread it. I would then recommend reinforcing the book by getting the CD or tape version. I bought it in 2000, just as my adult daughter started giving me fits and hassles. Gradually, I applied Dr. Herst's ideas to my situation.
I also recommend the highly acclaimed Boundaries by Henry Cloud and John Townsend. I finally recommend God Help Me! These People Are Driving Me Nuts! by Greg Popcak.
Why all the reading material? If you are a grandparent in the situation where you think your grandchild or grandchildren is in real danger from from your adult child, the grandchildren's parent, then you better know whereof you speak.
Dr. Herst brings her own experience to the party, a teenage daughter who planted marijuana in Dr. Herst's flower beds, and was taken to court. "Mother, get your act together," the judge intoned from the bench. Why? Dr. Herst hadn't planted the weed. She again mentions a group session, where she admitted she had some problems with her daughters. The group turned on her, telling her she "must" have somehow caused her daughters to react through neglect, even though they were the most loved, cared for children in the world.
Drs. Cloud and Townsend mention a couple who came to the office seeking advice on their grown son. The son had hot-and-cold running access to cash, a nice car, and admission to a new college when he blew off the prior, all funded by the Bank of Mom and Dad. The therapist told them flat out that the son had no problems, but they did. They were the ones with the bills, with the responsibility, and the son seemed to live a charmed life. "Would you like me to help you to give him some problems?" the therapist asked the couple. At first, they didn't understand. The therapist was willing to give them the tools to make the young man hoe his own row.
Dr. Popcak, despite gripes about his views on raising children, makes a very valid point in his book. People who are out to give you problems need to be forgiven as the Church teaches forgiveness through the Sacrament of Reconciliation. We don't waltz into confession, tell Jesus through the priest our sins, and expect Jesus, and the Church, to just say it's OK, we all make mistakes. There is a penance, an act, no matter how small, that we perform to show our good intentions. We not only have to say we are sorry. We are expected to prove it. Why not then expect those who have wronged us to prove they have no intention of sinning against us again?
Your adult children who behave in a manner that disrespects you and their children might or might not have what we call in our house F You Syndrome. Yes, they just might be out to get your goat. Yes, they may or may not blame you for whatever little unhappiness has happened in their lives. Yes, they might be resentful of their children, the little people they procreated. And yes, you will feel guilty, as if you caused it. As long as you give into that guilt, without taking steps to stop the cycle, the adult child can manipulate you six ways to Sunday.