Guest services will print you a ticket for a time that's available. I don't recommend it for smaller kids. My 6 y/o was very bored. It is strictly informational. You are allowed to take pictures. They go over each of the sections of instruments on the bridge (Comm, Navigation, Safety, Propulsion, the history of the ship). The tour is an Overlook Tour, which means you are on a "bridge"(on deck 9) that wraps around above the bridge (on deck 8).
You must remember though that the only time the tours are given are while in port (for security reasons). I think the times were like 12:30 to 3pm every half hour, but I'm not positive. We had 2:30 and changed to 2:00, but I know he had times for my group as early as 1:00. So book as soon as possible, but check your navigator to ensure it doesn't conflict with something else you want to do, if possible. It wasn't a problem for them to change the time of my tour, 6 hours after we set sail.
Hope that answered your questions.