JDiver

AcaMaca, take your own infant seat - good protection on the airplane if it's qualified, and you are not likely to find a USDOT approved infant seat in Mexico. Even if you find an infant seat - it will NOT likely meet USDOT approval, and may be more eye candy than safety.

May is low-shoulder season in Cancún - as well as the hottest month of the year in most of Mexico. Did I mention it will be HOT? Your best bet is to rent from the USA, using a major US franchise company (though, remember, a Hertz or Avis in any other country is NOT US Hertz or Avis...) and for anywhere on the Yucatán peninsula Apr - Oct, be sure it's air conditioned, especially if you have children with you.

BTW, unless you're planning to drive out of the area to take in ruins, etc. it may be much easier - and often even cheaper - to hire a reputable driver and well-maintained taxi for a day. For trips to town, the Isla Mujeres ferry, etc. a taxi will do fine, and local street busses run frequently and cheaply. For a day trip to, say, Chichén Itzá, I'd take a deluxe bus or a tour in a nice, air-con bus.