Sorry OP. That sucks. My kid had tonsils out and that alone is enough to deal with.

She is lethargic because she hasn't eaten in days. I would focus on getting real food into her if you can. Soup, ice cream, soft mac and cheese. Anything. My experience with little kids is after even one solid meal, they bounce back quite a bit in terms of energy level.

Apple juice is a bad idea - too acidic. That probably hurts her throat. Switch to dairy based cold things.

DD had her tonsils out and then promptly caught a stomach bug. So day 1 and 2 were recovery from the tonsil surgery (went great) then started throwing up for 24 hours on day 3. We are on day 4 post-surgery and she is finally not throwing up but is super lethargic. I assume the stomach bug set back her tonsil recovery quite a bit. Any idea what I should expect in terms of recovery timeline? She is drinking water, diluted apple juice and having small bits of food today. I am worried she is going to lose a lot of weight at this rate (only 38lbs to begin with).

And yes I have been in contact with her pediatrician and surgeon. They both told me that the throwing up was most likely not related to the surgery and that she probably caught a stomach bug. We stayed overnight in the Children's Hospital after the surgery so I am guessing she caught it there, but it could have been from anywhere.