car seat on a plane

We are flying from NY to Orlando to get on a cruise. Is it mandatory to bring a car seat for my 19m old? We won't be using it at all once we go on the cruise. (Taking the Disney bus to the cruise right from the airport). I saw online a kinda seatbelt for kids that can be use on the plane. Is this a real option?

Extra info: I'm buying my daughter a ticket for the plane.