I've traveled to central and south america extensively. Chemical cocktails are necessary if want to avoid getting bit by mosquitoes, but when in and at the ocean bugs shouldn't be a problem at all. In looking for a bug repellent, search for the ingredient DEET. The more DEET, the better. I would recommend bringing light weight long pants and long sleeved shirts for evenings or other times they are not in the water.
Whenever I travel to these parts, I also always take anti-diarrea, anti-nausea, and anti-biotics. I've rarely had to use them, but when I did, I was so glad I had them nearby.
And like another poster said, make sure they are diligent in watching what they eat. No raw fruit or veggie with the peel, no fresh salads, and no ice. I also tend to stay away from any meat that's not chicken. Drink PLENTLY of water and limit sodas and candy.
I hope they have a lot of fun!