Coming from a guy who has travelled with my 2 daughters as a single father since my now 10 try old was 14 months old.. the single most important item, you said you already have, is 90+spf waterproof sunblock.. put i on every time you think about it.. I have always been ocd about this.. and not much else.. I worry about a room when get somewhere, sunblock I have planned.. nothing worse then seeing little ones suffer from blusters and burns when it is totally avoidable.. and I've seen so many kids burned over the years when parents don't get the uv is stronger in the carribean..
Otherwise, sounds like you have a good start.. there is so much activity in Negril, you won't need as much entertainment as you might think.. my kids love Negril.