PMSL Are they being serious?
Now I'm an acknowledged camping/roughing it wierdo and not even I would go there for Christmas or while heavily pg and with a 3 year old and as for both together.....
It'll be cold and uncomforable and miserable for you.
What does dh/dp say about it? Surely he isnt in favour? Just say no and mention early labour if anyone tries to gainsay you.