No, tell them as much as they need to know. There’s a big mystique about cancer- you wouldn’t worry about telling them if you had another illness. Your cancer is treatable with radiotherapy, which is great. You will feel tired during and after RT so it would be good to let your grandchildren know that so that they understand they may need to be quiet and helpful, and you might not be as energetic as you usually are. Cancer can be frightening, but if we are open about it- an expert suggests using “I have a cancer” rather than I have cancer”- it makes it less scary, and maybe encourages more people to seek treatment early.
Good luck with your RT, I found it was fine, and I hope you are recovering nicely from your op.