Involving girlfriend in sons life but ex and gf hate each other
So my girlfriend and my ex "sons mother" absolutely hate each other. My sons 4th birthday is coming up and his mother and I have 2 parties planned. One for his little friends, one for our families and friends. My girlfriend feels that I should be including her in the "kids" party although her and his mother have never spent more than 1 minute in a room together. Her view is if I ever want to move forward with our relationship she should be involved in these events.
My opinion differs, I told her I'd like her to be at my sons family/friends party where there will be more people around to ease the tension. I don't feel that my child's birthday party with his little friends and their parents "who we don't know" is the place to start that part of our relationship moving forward. I feel that my son will feel the negative tension and it may put unneeded stress on him.
What do you think? Please help!!!