Hi, my dd2 had an adenotonsillectomy aged about 25 months.
Can I start by saying that I had amazing tips and hand-holding from MNers over on the Childrens Health section: if you do a search on 'tonsillectomy' or 'adenotonsillectomy' in that section, you will find endless pearls of wisdom 
But in a nutshell, I found the best tip to be: stay on top of pain medication after the op. Give next dose a few minutes before its due, and keep it up day and night for at least a week. Pain is far easier to manage if it never gets 'the upper hand', as it were.
dd was very young, and refused to swallow anything for 48 hours (bar ice lollies and, bizarrely, occasional crisps) so we got suppositories of Voltarol and paracetamol, which were a godsend. But I think its fairly unusual to have that amount of pain - she also had a minor larynx op at the same time.
Take change of tops for both of you, and clean knickers for her, because kids sometimes throw up after anaesthetic (dd didn't) and its very hot and sweaty in hospital!
Take drinks and snacks for both of you, to save you having to leave her alone if she needs you and to give her a choice of things to eat/drink. Rough foods are GOOD - they clean the wound, and lower risk of infection. Toast, crisps, etc. Things that stick to the wound hurt, so cheese, pasta, mash etc - not so good. Orange juice, obviously, stings. Sqash is good. Ice lollies are great for rehydration in toddlers who won't drink, and soothe to boot.
Books/toys/dvds to distract after op.
She may be confused or upset when she comes round from anaesthesia - tbh, I found this one of the most upsetting things with dd, because she was wild when I got to the post-op room. Really hysterical. But needless to say, she forgot about it within minutes - it was me that remembered! They said it was a fairly normal reaction to the anaesthetic (possibly along with waking up in strange place).
It can be normal to have a slightly raised temp the night following the op - I was worried about infection, but when I phoned the hospital they reassured me (why not before?!?) it was common. Temps over 38 though, or any bleeding at all, go straight back - they will tell you that anyway.
Around the 5th day post-op can be more painful - just when you think its eased and they are happier, they get grumpy again: the scabs are shrivelling and starting to fall off. She may get smelly breath (dd didn't). Keep up the pain meds.
It is normal for kids to be easily tired for anything up to 3 weeks post-op - its a general anaesthetic, then pain and painkillers, so utterly normal. I remember dd being pale and weepy/grumpy well after being out and about again - just convalescence
Good luck, its amazing how much this op can change lives - hope it all goes smoothly, and don't hesitate to post again if you need to.