Maria,
Best of luck with your surgery next week. I also have two small children (3 and 6). My 6-year old has overheard a few discussions, so I just explained to her that I have a "hurtie" in my ear and I will be going to the doctor to talk about how to fix it. She was satisfied with that explanation. When it comes time for my surgery (not 100% sure, but I'm heavily leaning that way), I plan to just follow up on that and tell her that they are removing the "hurtie" so that I will feel better. I think the important thing is to try to be honest, but with an age appropriate explanation that isn't too scary.
I wish you the best of luck with your surgery. I don't blame you for being nervous, but us Mothers are a tough breed and I know you can do it. You will do fantastically well, and then you'll have some relief from your terrible face pain and get back to a normal life.
Lyn