You poor thing, to be so worried over it all. Try and answer your own questions, though...
What if the toys they want are sold out? You'll get them something lovely and different, or an IOU.
What if they see that it's you buying the presents, not Santa? You fudge it! You tell them that Santa brings the stockings now they're this age, and Mum does the big presents.
What if they have to come with you to the shop? Same as above, you tell them it's a special trip to get the presents from Mum. (Not easy or ideal, admittedly, this one, but could work esp if you can click and collect).
What if they eat all the Xmas snacks? They can't if you hide them :-)
Etc. etc.... The point i'm trying to make is that the anxiety is often far worse than the reality of the upcoming scenario, so try to force yourself to imagine what will actually happen if the things you're fearing do happen.
You sound like you are doing your best to give your kids a lovely xmas, and you sound emotionally exhausted. There are many of us in that boat 
YOU'VE GOT THIS!!!!