I would NOT go to this. That is me.
I would not drive all that way, for a birthday party, relative or not.
PLUS, it is bad timing per your own family's obligations and your own child's b-day. And other holiday stuff too, to do.
And, if you all do go, then that also means, that you or your Husband or both would have to take off of work, therefore using up your "vacation" time, too, right? (I don't know if you and your Husband work, just assuming).
And Employers, need to know AHEAD of time, of any vacation time their employees want.
I would not go, nor put myself or my family, in a crunch like that... not to mention the "budget" or money expenditures for it and traveling.
Plus, you don't even know, IF going to visit them after Christmas, is okay with them, being you'd be staying at their house. And it is after, the party and after the holidays anyway.
I would not go.
Are they coming and traveling to YOUR house/town, for your birthday and your son's birthday?
Ditto Marda & Hazel below.
You and your Husband need to determine, HOW you will use up your "vacation time" or time off from work. Do You-- want to keep it for your own family/kids, or for going to something that you feel "guilty" about if you don't go???? And you need to determine, if EACH year, you are all going to travel out of town, for all the relatives birthdays and their parties. Plus, dealing with the regular Holiday gatherings, aside from B-days, and your own holiday obligations you need to do, too.
You either "spend" your vacation time and money, the way you and your family needs to... or you do it to feel less guilty to please the relatives.
We personally don't expect relatives from out of town to come to our or our kids b-day parties. Regardless of their ages.
So just a story: recently, my MIL told my Husband that they are all going to his brother's house for Christmas. BUT, she adds "everyone will be here except you/your family...." and made him feel guilty for it. Mind you, his family lives far away. And going to visit them, costs US... an arm and a leg and we simply do not have the money or time..... to go visit them. But MIL does not even, think of the hardship it is for us. And, I told my Husband, I cannot... fathom, spending all that time and money... to go travel to see them, just because his Mommy, makes us feel guilty about not going there. I mean, c'mon. I said, he can go alone if he wants. We simply do not have the time or money, for ALL of us to go fly over there anytime they want us to. And we should not feel guilty about it. We cannot go into financial or emotional "debt", just to please them.