Infants and toddlers on airplanes are safest in a car seat with a harness, due to the risk of turbulence. A child who rides in a car seat on the ground should ride in the same car seat on a plane. Air turbulence can be dangerous and can appear suddenly without warning.
Not all car seats can fit on standard airplane seats, which are typically about 16 inches wide, but Safe Kids Carroll County and the Federal Aviation Administration strongly recommend using a car seat if it fits. As in cars, babies under two years old are best restrained in a rear-facing car seat, and a forward-facing car seat can protect toddlers up to 40 pounds or more. Make sure your child’s car seat is labeled “certified for use in motor vehicles and aircraft.”
You need your child’s car seat to travel to and from the airport anyway, and car rental companies might not have reliable car seats available. Children are better off in their own car seats.