P.P.
Hey T.,
gee.. I don't think you are a bad person for having second thoughts. I think most people would understand that you are a bit apprehensive to attend a family only party of someone you hardly know. Perhaps you could email or call her and tell her just want you said in your post.. about how you are feeling really overwhelmed in getting ready to go away for the holidays and will not be able to make it. Really.,,. its that easy.. no harm in that.. perfectly acceptable.
OR.. You could say after talking it over with your husband, that you are not comfortable driving down there by yourself and your child. OR you could mention both reasons.. whatever you are comfortable with.
I think it would be nice maybe if your budget allows to tell her that you would like to send her child a small gift in the mail to let her know you are thinking of her son or you could offer to set up a play date at a halfway point for both of you... after the holidays so the kids could play and bring a little gift (a book,, something simple..)at that point.
Hope that helps.. you are a lot like me. you don't want to hurt feelings but I think its important to do what you are comfortable with!! don't worry about this anymore,, be gracious and honest. If she gets mad.. then she isn't someone worth getting to know, in my opinion.