I took the tour with my 11 and 13 year old and had a great time. The sharks are nurse sharks, and the rays are very gentle. The tours regularly go here.
We used Coral Breeze Limited for our tours and highly resommend them. They are very accommodating, and can be easily found. All information is on their web site at http://www.coralbreezelimited.com
Don't be afraid, and enjoy yourself. You'll be fine and have the time of your life.