Our son's went well. He had severe tonsilitis and the doctor recommended removing both his tonsils and his adenoids (even though they were affected) at the same time, saying in time the andenoids would have to go anyway. So you might want to ask about that considering your feelings about surgery. It may be well worth it to only have one surgery and be done with it.
Recovery lasted about 2 weeks. The first week was just down time and healing, the second week was scab watch for the tonsils. For some patients there may be bleeding when the scab falls off.
After that, smooth sailing.
I don't know if there are diet requirements for just adenoid removal. Since ours had his tonsils out he couldn't have milk products (causes phlegm which is problematic after surgery) and of course nothing sharp, acidic, or spicy.
Keeping them resting and quiet will be the biggest challenge. Before surgery round up movies, any special foods, and quiet games and activities so they won't run around too much.
As for the concern about weight gain...no problems here. Our kid is as skinny as they come and remains stick-like to this day. He had his surgery two summers ago.