When we were down in November our 3yo alternated between a puddle jumper http://www.coleman.com/puddlejumper-child30_50lbs/
and a regular life jacket. The puddle jumper seemed to be the go-to for hanging out on deck and the regular if we went in the dinghy. The puddle jumper is coast guard approved though so it would be fine in the dinghy as well. The older children (5,6,7,9,11) all wore either a regular or neoprene vest.