As YP says... The J/S pax have elected to pluck the 10 year old child from a perfectly good seat and place her on the floor. The CC would have been wise to stop this.
Also, a GH manager should have met them on arrival with arrangement for a refund. I'm sure they would have been happy with that, and the airline would have saved the massive denied boarding costs. Maybe the crew didn't organise that?
Either way - much safer (as a crew member) to simply deny boarding and let the airline pay. Doing a passenger a favour could cost you your job, even if what you did was legal and helpful.