Baby sims, especially ghost sims can die when you are not playing them as an active household and most of the time their pictures do not gray out as they are technically dead, but living dead. I usually get my ghost sims turned back to living sims as soon as they age to child, because ghosts really die easily when they are born in game and don't play that household.
Keep in mind we can't kill baby sims and toddler sims, but the game can and does. if your town population gets near it's population limit the game starts killing off all the babies and toddlers - with ghost sims first. You usually discover they had died in an accident or starved by having your sims read the paper every day as their death are in the paper the day after they died. Also there is not urns and things for non- EA sims that die when you are not playing those households. So if you missed the paper notice, then you keep looking - but it does you no good. Once they are gone they are gone for good. The game takes them right to the Netherworld and they cannot come back period.