If the problem is not directly with your ex daughter/son in law and yourself, you could always send an email/letter to them, I would avoid direct contact with the children, this could make the problem worse. But doing something rather than nothing has to be good.
I have been in this situation for about 4 years on and off.. I am the daughter in law but I am with my hubby, the relationship in question was to do with mother and son. I would never stop gran parents seeing my kids as I had it as a child and your just hurting your own children really. I don't know your situation so it is hard to help but do not stop christmas and birthday gifts/money that really peed me off lol.. We have managed to sort out our problems. I hope you do in time.