OP -
great thread!! i too have noticed that these threads have started already!
i can see both sides, if your parents/inlaws live hundreds of miles away and you have young kids and no room in your house, then YANBU at all to want to spend it in your own home with "your own little family" but an effort should be made to see eachother at some point over christmas/new year.
i think it all depends on the parents and inlaws situation. for example, my mum is on her own, she has a partner but he spends christmas with his brother and his brother's family in another city. my mum can't go due to work. she doesn't really have anyone else to spend it with, so there is no way on god's earth i am leaving the woman who gave me life on her own on christmas day. she comes with me, either out to lunch or to my home for lunch. she's my mum and i adore her.
MIL has FIL and BIL (he still lives with them) to spend the day with, so i don't feel guilty about not spending the whole day with them. but i would feel guilty if i didn't see them at all on christmas day (although this wouldn't happen, DH would never allow it, they are his parents and he would want to see them christmas day, plus i want to see them too), that wouldn't be fair, so we invite them for christmas breakfast and see them for a couple of hours that way.
if your parents and inlaws had alternative arrangements, going to friends, had extended family going to theirs etc... then it's perfectly reasonable to say "you know what, we are just going to have it on our own in our house this year with the kids, but you are of course welcome to call in for a visit if you want" personally, i don't think that's what christmas is about, i think you should celebrate with as many of your family and friends as you can over christmas, but each to their own.