I've had this happen several times, particularly when the parents were relatively old when they had the kid (or kids in some cases). Although the orphan can't work, provided there's food available they should be able to keep going without problems until it's time for them to go to college, when you'll have to start a new generation. Although they can't do any work, as I said, they can still earn some money by finding duplicate collectables, so unless the family was very unlucky with money, there should be no problem paying for the food.
Of course, you might choose to start a new generation straight away if you want, when the orphan will automagically get old enough to start work, and will be available with his/her siblings for selection as the next house owner.
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To err is human; to arr is pirate.