I don't know anything about the bbq's in Cane Garden Bay, but we took our kids a year ago (20 somethings as well) and really enjoyed eating on the boat. After a full day of sailing, snorkeling & swimming they weren't in a hurry to go out. That being said, while I don't remember the exact $ amount, the jumbie BBQ was one of our pricier dinners of the week (although that could very well have been all the painkillers we drank from the bar that night too!).
You might consider leaving the kids on the boat one night and having a "mom & dad only" dinner out at a special place!