You can't always take age into account when planning a trip. However, I have some advice for you. This trip would be the most effective if your children are still in primary school. At this age children feel comfortable around their parents, and they like the same things as their parents. When they're older they need their own space. Thus a small boat is not the ideal place for them.
The schoolwork on secondary school is harder than in primary school. When you're in the Caribbean you don't feel like making homework all afternoon. Even though you spend less time on homework than you would do at home. Having to spend much time on homework will be demotivating.
Socially teens will miss something. At school they have friends and they will miss them more at this age.
Children should definitely not miss the first year of the secondary school: If they start in the second year they will miss the startup of friendships.
As from the age of seven they will remember most and will remember this unique trip.
It doesn't mean they can't enjoy if when they're older or can't remember it when they're younger. If you listen to the interview of Joost ter Burg, you'll hear he really enjoyed the trip he made the trip as teenager.