It is definitely frustrating that the Marriott website logic for enforcing max. room occupancy sometimes doesn't handle children correctly. In your situation, I would search for two people and as long as the room permits cribs you should be fine. To be a %100 certain, I also recommend emailing (or calling) the hotel to confirm the reserved room is fine for parents plus an infant in a crib. Yes it is more work and you shouldn't have to do it; however, I don't know of any other workaround. If you know the property (from previous stays, etc.), confirming in advance probably isn't necessary.

--Jon