We are GPs to a 2 yo and 6 month old; last year we bought a portable DVD player for the plane ride and beach time for the 2 yo when he was 12 months.
Also, a cotton sheet is good to have for them to nap on the beach under a palapa. EZ to shake, wash, & dry each night for next day use.
Foldable beverage bags (large w/compartments & /zip lock bags for snacks & for holding ice (re-freeze em overnight to save water)
Sunscreen is a MUST!
Some great Tips
We stay @ La Cabana; here is their
Children's Activity Schedule - geared mostly for older than 18 months tho
Enjoy!