A.A.
You could try a girl's luncheon and then say you had to bring your son at the last minute. That will only probably work once though--you are probably just gonna have to get your big-girl panties in and tell her the truth. It stinks, but I'm assuming you have a valid reason for leaving out grandpa and gramma deserves the truth.
I just wanted to edit my answer because I went and looked at your profile and see that the reason you don't want any contact with grandpa is that he is a repeat child molester! OMG---I am so glad you do not let your son have contact with this man. But seriously, why would you want him to have contact with gramma either?!? Even if she wasn't the abuser, the guy has messed with at least 6 kids in your family---she cannot be completely in the dark. She has to know about the past and in my opinion that makes her just as disgusting as him. She deserves no time with your son either. If I was you I would count my blessings that you found out about this and steer completely clear of this couple completely! This totally sickens me that this guy still has a family and a wife.....