All the DM pictures show a child sat on the floor. I’m sorry but the child was given a pax seat but the parents put said child on floor in flight, presumably because they wanted to sit together, and for the picture opportunity.
They could have all have seats during the cruise. The parents could easily have sat at doors 3 jump seats during cruise and the service. I’m sure this would have been offered by crew, but the parents probably declined so they can all sit together on the floor.
The aircraft was certified to carry said number of passengers, whether in pax seat or jump seat. Nobody was without a seat opportunity at any time.
I doubt the crew forced the passengers to do anything. They would have been given all the options, including off load. No safety or security has been breached here. Compensation is a completely different matter. IMO of course.