We did the trolley in Key West with my kids when they were 11 and 8. They were kind of bored for the driving/talking part. We mostly heard about the history of the area, architecture, that sort of thing - which they aren't interested in. At one point we got off, and then it took for-ev-er for the next trolley to arrive. It was a little ridiculous. We could have walked back to the ship faster! Also, I oversold the whole "trolley" aspect. My kids quickly noticed it's just a bus that's been made to look like a trolley, LOL! So overall, it wasn't a great experience with kids, for us anyway. They had some other little train thing that was cuter and might be better with a 6yo.
They both really liked the little aquarium, if you want a fairly brief activity. Easy walking distance from the ship, right at the main trolley stop. We were there maybe an hour? They had rescue sea turtles, a little touch pool, sting-rays to touch, and a nice gift shop. If I had a do-over, I'd just go there and skip the trolley.