K.M.
I think it's time to question your boyfriend why he's so concerned about his ex. Yes, he needs a relationship with her since they share a child, but his main concern now should be the children and YOU.
If his son wants you to be there, you should go and not worry about this other woman. This is effecting YOUR relationship with the children who might be your step-children someday.
It sounds like she might be manipulating the situation so that you can't have a good relationship with her children. (Although I realize, as a mother, it would be very difficult to let another women into your childrens' lives.)
Go this weekend and enjoy your time with your boyfriend and his son!