We have not taken a taxi to Coral world, but we have taken taxi's other places and never had a discount for extra people. The prices we were quoted were per person, age didnt matter, and for one way only.
Be sure and ask your driver before you get into the cab for the price up front, as some people who dont ask have got ripped off. Of course they usually want a tip too. But that is up to you. The shore excursion desk on board or your shopping guide will be able to tell you what a reasonable price for a taxi will be. Sometimes they will even recommend a certain taxi because they have a good reputation for being fair with their rates and a good safety record. Some of those drivers are really crazy. Especially on St. Thomas and St. John's island. So be careful.
mom x4,grandma x4