So if OP's mom is anything like my grandma (and I suspect she might be), here's how I see the scene playing out.
RegionMom and crew show up at the house sans cookie making goodies. At which point her brothers ask "Region, where are the cookie goodies?" to which Region responds "Well, mom said she only wanted us to do trains, not the cookies", so brothers look to mom and who says "What are you talking about Region? You do the cookies every year! Oh the little ones were so looking forward to it, how could you not bring the cookie stuff?", at which point Region blinks...didn't she? Wasn't there a conversation about? "Mom, you said in our talk about the rules, you didn't want Santa cookies this year. Remember, we couldn't bring our dog, we better not wear shorts, and no cookies?", which will get a "What are you talking about. I never said any such thing! You didn't bring the dog? But my little precious was so looking forward to playing with your dog! And now there are no cookies for Santa either?" at which point the toddlers begin to cry because there no cookies for Santa and he won't come if there are no cookies, everyone knows that. So Region feels guilty and crazy because she knows she had that conversation...right? And because she can't sleep over the guilt she ends up wandering around the only 24hr store in the area in her slippers mumbling about Santa cookies and no shorts and the next thing ya know the store is calling the cops and it becomes a whole thing.
Of course if she shows up with the cookie stuff without telling her mom about it, mom flies off the handle about how no one listens to her or respects her or loves her! She then locks herself in the bathroom, eventually she finds her way out, knocks back a couple of egg nogs and replays every second of how her children never appreciated her and blah, blah, blah martyr, and more sobbing, locking in the bathroom. And again...Region ends up feeling guilty.
I love my family very much. But we're nuts. Every last one of us. The key is learning to ride the crazy wave.
So my advice would be a phone call. Hi mom, Region here, how's the dog? Well I was just calling because I was talking with brother and mentioned how we're not doing Santa cookies this year and he seemed really confused as nothing had been mentioned to him and he was looking forward to do the cookies. Since he didn't seem to know what was going on I just wanted to double check with you that you don't want them done this year, right? I'm willing to put down money that mom's response will be "What are you talking about? Of course we're doing Santa cookies!", at which point you say "Oh, ok, I must have misunderstood. So I'll bring the stuff to make the cookies then. Great, thanks mom!" and hang up.