My daughter was 29 months when she had tubes, adnoids, and tonsils. She also has sleep apnea and seasonal allergies.
Here's what I can tell ya: yes, it does help. Yes, it does hurt. My daughter freaked out when she woke up in recovery. The IV was a big problem for her. BUT, she had 2 popcicles and a half a sippy of apple juice before she left recovery. We had the surgery early in the am, but we had to stay all day and night at the hospital (Dell Children's) because of her age and her sleep apnea. She did really well. She ate ice cream, mac and cheese, yogurt, jello, and tons of chicken noodle soup (lukewarm). We brought her princess/Disney DVD's and she watched movies and played with her babies. They had a toy room for her to go to as well. She didn't regain her voice for 3 days, and she didn't sleep at all the first night. For like a week afterwards, we woke her every 4 hours (I believe) for her prescribed medications and pain relievers. This sucked (for all concerned), but we got through it. She started to get frustrated toward the middle of the second week because she really wanted to eat chips, granola bars, and goldfish and couldn't - we wouldn't let her. But she wanted them which meant she wasn't feeling too much pain anymore.
As far as her symptoms go, she's had 2 ear infections since the surgery. BUT both leak out green nasty gunk from her ears, which means the gunk isn't hanging out in her middle ear. Her hearing loss has gone away. She doesn't snore or toss and turn or stop breathing like she used to. Oh, and I should note that both ear infections followed not giving her the daily dose of allergy medicine that she needs, so that's more our bad than anything. She hasn't had a single case of strep since then though! :)
To address your concerns:
pain - you'll be given some pain meds and then you'll move to the over the counter - just stay on top of it and she'll be good. Her throat does hurt though, so baby her lots. :)
dehydration - keep lots of good tasting liquids around the house and encourage her to drink and eat what she can and she'll pull through just fine. sherbert, yogurt, and apple juice (which we don't normally do) really were treats that she enjoyed. We'd remind her to drink whenever we thought about it. The more she drinks, especially right away, the better it gets. The longer she goes without swallowing, the worse it'll hurt to swallow - so I hear.
bleeding - the blood loss during surgery was so minimal it was really amazing. Your best bet is to remember not to give her anything that thins the blood prior to surgery (vitamins, tylenol, tea). Ask your ENT for a list. After surgery, I never saw any bleeding.
Oh, whole surgery start to finish for both ears to have tubes, tonsils and adnoids removed: 20 minutes. Recovery: 30 minutes (I was with her though). :)Good luck!