why? The parent choose a ticket that does not include seat selection. The website is upfront that it does not include seat selection. People choose and pay for seating for a variety of reasons. We do so so that we sit with our children. We also check the seating on a regular basis to make sure plane changes etc do not displace us. Its personal responsibility. Will Delta try to help, probably. But the OP put herself in this situation.
Sorry, but I disagree. You have to enter birth dates when you make airline reservations these days - airlines either shouldn't sell a booking if it involves the possibility of a 3 year old sitting alone, or if they sell it, they should have to deal with the outcome and make sure a 3 year old isn't sitting by themselves.
Corporations have responsibilities too.
As a side note - airlines will often show no seat assignments available when you look at seat maps online, even when there are plenty of seats still unassigned, to try and scare you into paying for an upgrade. See this article: http://www.wsj.com/articles/the-airline-fee-to-sit-with-your-family-1464198317 In their example flight, 30% of the seats were unassigned, but the airline was showing nothing other than seats you had to pay extra for.
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