Here is a nice web site that lists resorts that will allow 2 adults and 3 kids in one room.
http://travelwithkids.about.com/gi/d...m/famfive.html
We just went through the same issue last month. We actually decided that a large all inclusive was not the way for us to go, and ended up booking the family suite at Playa Maya.
Hope the information helps.
JT