DS1,now nearly 9, developed a hydrocele at 6.5 and had the operation at 7. We were referred by our GP to the urology clinic at Chelsea & Westminster, which took a couple of months. When they saw us they asked if it was growing or seemed to be getting smaller, as sometimes they resolve spontaneously, apparently. He was booked in for surgery a couple of months later.
The surgery itself was very straightforward -a day case, but nothing to eat from the night before and nothing to eat or drink on the morning of the operation. It was pretty quick (no more than 30 minutes), I think, though DH took him. He was cross and groggy afterwards and hyper from the anaesthetic later.
I made sure DH took snacks and drinks for when DS1 came round, as it is not always a mealtime, and he had been ravenous after his last day surgery (something else minor but unrelated).
There is a scar of about an inch on his groin. This was painful for a day or two (they sent him home with ibuprofen for the pain) and a bit sore for about a week - he didn't do PE and wasn't allowed to swim for a week. It has healed nicely. They saw us in clinic again about 6 weeks later (can't remember) to check that it had healed and that the hydrocele was fixed.
Hope this is helpful and a bit reassuring. The prospect of surgery, even minor, is frightening, and I was really nervous, but it went without a hitch.