I think I am going to have to weigh in on the "wait" side as well. I took my daughter to Germany when she was 8 and that was a great age for that type of tour. For our Alaska trip this summer, she was 9, and I think she was too young for that trip in many ways, as it there was alot of more sedate activities, fewer magical touches, and she didn't care much about scenery or waiting for wildlife.
I think you have to think a little selfishly as to not only how the child will enjoy the trip, but how YOU will enjoy the trip if you have to placate a child who can't keep up with the tour or with the older kids.
Our guides would often sit at the kids table while the adults had a nice adult meal just a table away. As a single parent, I greatly appreciated this! But had my daughter not been old enough to take care of herself at mealtime without adult help, I would have probably been off to the side sitting with her or sitting at the kids table, which definitely would have reduced my enjoyment of the trip.
The other thing to think about is that there likely won't be other kids that young on the tour for yours to play with.