My 5.5 yo daughter (with Autism)had tonsillectomy/adenoidectomy and ear tubes put in Aug 3rd in Durango. We were also very apprehensive, especially since her anxiety was VERY high. She came through surgery fine, but the recovery room was very very scary for her. The nurses were very gentle (they all knew about her disability) and tried to keep her calm. Since we had to drive an hour to get home, they gave her a low dose of a sedative to keep her comfortable.
After she had slept everything off, she got out of bed, tapped me on the arm and said, "Mom, I want hot cheese sandwich, please." (Grilled cheese) Well, of course I gave it to her. We had a few problems with constipation, and the third day she took a small dive (refused even popcicles for a few hours), but otherwise her recovery was exemplary. We were so worried, because in addition to having Autism, she also has Celiac Disease (no wheat, oats, barley, rye) so her "soft foods" diet after surgery was a bit limited. We were also concerned b/c a friend of ours who had twin girls w/ Autism had their tonsils out at 3 yo...one of them refused fluids for ten days and had to be sedated at the hospital w/ an IV (would pull IV out if not sedated). I think that is the king of worst-case senarios, though.
Let your child be the guide. My daughter was eating sandwiches and cheetos by day 5, and pretzels two days before her post-op appt. But not all kids are that way. She was SO sick b4 the surgery that I think she was relieved to be able to finally EAT! Good luck!