Hey Linda
We took our son to Cuba twice already, and he's just turned 4. (Of course, we had him outnumbered, both times ;)
The lifejackets are perfectly reasonable. Saw lots of kids wearing them, both at the beach, and in the pool. I suggest, when you get to the airport, going to the departure lounge as soon as possible. The concourse is boring for kids, but from the DL, they can watch the planes being loaded and unloaded, and moving around.
For the flight, hard candies are a must ie lifesavers.. sucking on candies will help them with their ears. (Bring lots, if your kids are like mine)
Hopefully, you'll get at least one window seat, for the flight, but I know many passengers will let a child join them, if they don't. This year we had a late flight coming back, and Logan had no trouble sleeping most of the way home on the plane.
Those little handheld games, that kids get in a Happy Meal, are perfect for a trip... small enough, yet provide losts of entertainment.
More to come as I think of it.
Cheers
Gambitt