I haven't read your story, so I am answering based on what you wrote.
Here's my analogy-- just because you don't know exactly *what* you are having for dinner, doesn't mean that you don't plan to eat.
If the kids are her responsibility that night, then it's her responsibility to find care for them. Why do you have to "justify" where you will be and what you'll be doing? It might be that you wanted to stay home and have a ceremony to send out the old and welcome the new and you didn't want the children to bare witness to any emotional outbursts. It's not for HER to judge whether your plans should be dropped to accomodate her.
As far as what I have planned; my H is in NY; I am moving to a rental starting Jan 1. I am going to be packing like a crazy person these next few days. My 2007 has SUCKED BIG TIME and not just because of the marriage going south. I have been invited to do things, but I think I want to stay home, and have a cathartic cry fest and ceremonial burning to say "so long and good riddance" to 2007.
Me-43 H-46 M 12 yrs 7/09 T 15 2 grown kids bomb 7/05/07 H moved out 8/04/07 11/22/09 told him I quit;let's get ball rolling Mid Dec- he isn't sure he wants D End 2/2010-Starting to consider piecing