totally understand the worry beforehand, I was petrified (dd was very thin, and most of that was from not being able to eat properly with huge tonsils). Honestly, the worrying beforehand is far far worse than coping with the reality - though of course its hard handing them over, when they go to theatre. Try and have someone with you, go for coffee/cake while she's in surgery even if its for ten minutes...passes the time.
I had dd in with me for the first few nights, and gave pain relief every four hours round the clock. She needed it, so woke anyway - others don't, it does vary.
Lots of DVDs, lots of TV, and sips of water...no juice, esp. orange, it will burn. Do expect pain to get worse again after a few days (5-7 post op) as the scabs fall off (ugh, I know, but its good!) and don't ease off on pain meds till you're past that stage...she may have ear pain at the same time, thats normal. If she develops a fever, or bleeds, contact the ward or go back to A&E: its rare, but infection or small bleeds can happen.
Take a book to read to her post-op in hospital, even if its for an hour or two (dd was in HDU for a night, as she was so little and had a floppy larynx which made her high risk for anaesthesia) or a comic...though she may not feel like it, she might want distracting. Loads of kids are up and about within a v short time, but make sure she rests and doesn't do anything to active for a few days, while the wounds heal - helps minimize risk of bleeds/infection.
Thats all the things to look out for, but honestly - you'll cope with it beautifully when it happens. Take it an hour at a time (like she will - kids are great at living in the moment!) and come back here to be reminded what a good mum you are for helping her improve her quality of life. Its made a huge difference to dd, am sure it will for yours