No Free Passengers
As far as plane rides are concerned, there are no passengers who don’t have to pay for a seat except under certain circumstances like being a Disabled Veteran. Airlines charge for pets, extra baggage, and children alike, as they all take up space on the aircraft. If you want a seat on the plane, best buy a ticket.
A parent booking a flight for them and their children left a review complaining that they had to pay for a seat for each child, as everyone must get buckled for takeoff, landing, and turbulent experiences. It is for the safety of each passenger that they have their own seat and seatbelt, of course, and not out of spite.