Vacation planning with children

I am starting to plan a family vacation for next year around April or May. I would like to go to the Caribbean and maybe to an all inclusive although I've never been to one before. It will be my mom, my sister, me, my 4 year old brother, my 3 year old daughter and possibly M (my fd) if she is still with us at that point. Have you vacationed at a really great place? Do you have any recommendations? What should I be looking for with so many small children?