Brandy works wonders! It is fab for bad stomach aches, mixed with hot water, but then again, if that fails, if you have loads, it will numb the pain!
I went to the Sol Cayo Coco in 2003, and brought 2 first aid kits. One was in the main luggage with scissors, large bottles, lots of plasters (With extra tape, it doesn't stick well in the heat) tweezers and pain killers.
I had a small kit in my hand luggage, with a few plasters in case needed on the plane. Small tube of anti-septic, burn ointment, Aloe Vera (Miracle stuff, available in Holland And Barret) for during the flight. (It's also a great treatment for eczema and sunburn) Also in the small bag, any medicines you normally take. I've got asthma, and take a preventer as well as a reliever in case there is a delay, and of course good old asprin, which also prevents blood clots during the flight. Keep everyone's medicines together, with spares in case you lose them.
Don't take scissors, or tweezers on board. My mum got hers confiscated from her hand luggage, and we went 2 weeks without scissors.
Also, be careful in cuba not to get any cuts. My mum dropped a glass on day 2, and the wound didn't heal up for the 2 weeks due to the heat.
Where abouts in Cayo coco are you going? It's a fantastic place, I've been twoce, and am going to Cayo Guillermo in June.