twentyfourseven, FS means full-service. Also at full service properties you would get free breakfast for elite+1 (resorts excluded).
It's been a while since I've been to FL and I don't have kids so I'm going to defer to hhoope01, dburns, and the others who have providing excellent input.
A couple of things - don't forget you can book 4/stay 5 (ie, 5th night free) on points. Also, Marriott lets you search hotels by category so if you can only go to a Cat 4 you can search Cat 4 properties in FL (not sure there are any Cat 4 in the Caribbean). And if you're shy some points for a Cat 5 if you decided to go to Cat 5 you can purchase points.
Cheers.