A child seat on a plane is an illusion of safety. If the plane crashes it won't help. The only reason to bring one into the cabin (and it is a good reason) is if you need one where you are going.
> Did you know that the safest place for your child on an airplane is in a government-approved child safety restraint system (CRS) or device, not on your lap? Your arms aren't capable of holding your child securely, especially during unexpected turbulence.
> The Federal Aviation Administration (FAA) strongly urges you to secure your child in a CRS or device for the duration of your flight
A child seat on a plane is an illusion of safety. If the plane crashes it won't help. The only reason to bring one into the cabin (and it is a good reason) is if you need one where you are going.