Hi Mom :)
Is this for your 2 1/2 year old that getting crowns? It will be hard to get her through the night if she's used to feeding but you can either distract her and soothe her for the one night, or you can try to give up night time feeds altogether. If you have a month, you could start now - and gradually get her off them. There are a lot of ways to do this.
If it's the child who's teeth had the decay, it might be a good time to get her off night time feeds - I saw a post here recently about tooth decay from falling asleep with milk - I don't know if that contributed to the problem with her teeth, but maybe view this as time to get rid of them?
I think at 2 1/2 (if I have that right) she'll benefit from the extra sleep and less disruption at night, so my advice would be to begin weaning at night. You can still breast feed of course in the day if that's your choice. That way too it's not as drastic a change.
I think it's very important to not have anything in the stomach during surgery. The general anesthetic relaxes all the muscles including the air way and the digestive ones - so contents of her stomach can come up and go down into her lungs. You'll probably have to sign something (I can't remember if we did but we signed a lot of papers!) saying that she fasted. I doubt they would consider operating if there was any chance for aspiration. They will fully check her before surgery, so even if she was by chance dehydrated (very unlikely I think), they will monitor all that.
We had our kids fast for surgeries as toddlers. When they wake up you just keep them distracted - books in the car, movies, whatever. She may sleep on ride in where it's so early. And so much to see at the hospital, she won't really be focussed on hunger. Ours did fine.
I was able to get my very bottle dependent child off night time feeds in 2-3 nights. He woke happy and smiling, I was sleep deprived, but it was worth it. It wasn't near as bad as I thought it would be. He fussed, never bawled. And my nerves made it through!
Good luck :)
** If you haven't weaned her at night by then and she becomes really upset during the night, what about driving around with a movie in the car or something so she drifts off? I had to drive my colicky baby around a few times - it's one night, and maybe at least then you have a back up plan if necessary? Just a thought.