Should I tell my child(ren) that his grandparents are seperated?

My mother and stepfather filed for divorce 4 months ago and it is now final. He moved out of my mom's house in March. Each time I ask my mom when they plan on discussing this with my kids, she keeps giving me the runaround. My 6 year old daughter has already had her suspicions since the get go when she realized all of his things were missing from his room. My son on the other hand who goes to my mother's house regularly has been completely oblivious likely in part because of his autism and the fact that it is more difficult to piece together the puzzle and also likely due to the fact that each time he goes over there my ex-stepfather (the only grandfather he has ever known) is there. Although at first I understood the need to handle the situation cautiously seeing as the most minute changes can have lasting implications for my son, I grow more and more concerned that the longer this goes on and we don't say anything, the more likely it is he'll find out and then he will not only have "lost" his grandfather but he will also have lost trust in the person he trusts most in the world - his grandmother. What should I do? Should I give my mother a timeline? Should I just let the situation progress naturally? Am I right to feel as though they are lying to my son because of the fact that my stepfather makes it a point to be there when my son is there? Or should I feel happy that he is making an effort to still be a part of my son's life? Why can't he have my son over at his new place if he really wants to continue to be in my son's life?