Okay .... I realize that it is VERY complicated due to the ex and her opinion; however, I think she is making a deal of this simply because it goes against what YOU want to do. Since opinion are like armpits and everyone has one ... (and yes, some of them stink) here is mine....
For pity's sake ---
YOU are the boy's Dad. It appears that you play an important role in your son's life and are not an absentee father.
YOU have rights and responsibilities regarding how your son deals with the death of his great-grandmother.
What example of fatherhood and leadership are you setting for your son if you bow down to every whim and desire regarding your son that happens to be different than what your ex-wife expresses. Especially IF you think that she is simply being difficult.
As a child I was deprived of attending a funeral for a loved one because my parents wanted to shield me from death. To this day, I wish that my parents had handled it differently.
As a parent, due to an unexpected heart attack, my young son was unable to say "goodbye" to his grandpa. He mentioned it often while growing up ... that he wished he had had the opportunity to say good-bye. I realize there is some difference because there was no opportunity. However, I wonder how it will affect your relationship with your son if you don't take him ....
1) YOU deprived him of the opportunity. Regardless of the fact that he will be told his mother was the one that didn't want it ... he will remember YOU are the one that didn't take him.
2) He will also remember that YOU didn't stand up to his mom about something that YOU felt was important for HIM.
Personally, I think that if at all possible find
someone whom your wife respects that can act as a mediator and help her understand the importance of this to YOUR SON. It isn't a game of tug-of-war when it deals with a child's emotions. I pray she is first of all a MOTHER that will come to understand that and NOT an "ex" that simply wants to disagree with her child's dad.
RE: the girlfriend's daughter. Absolutely DO NOT take her if you can't take your son!!!!! Regardless of how it affects your relationship with the girlfriend --- she isn't your wife yet and right now, your relationship with your son is more important. If your girlfriend can't understand that ... then in my opinion, SHE has some growing up to do and may not be ready for the combining of two families.
Now.... I'm outta here.........