Sorry, but I take a different view.
Even if the flight was booked before the allocated seating came in, until the flight closes it is still possible to reserve your seating, for a price, and the pre-flight reminder email from Ryanair would have told her that. That being the case, all the mother had to do to ensure she sat with her child was to do just that - pay for seats and hey presto - no issue, no story, no furore. She chose to pay for tickets, she possibly chose to pay for hold luggage, so she could have chosen to pay for seats. She didn't. End of story.