Can a child sit in a regular seat or do they have to use a car seat on your airline?

Airlines have differing regulations regarding the seating of children in aeroplanes.

You should check with the airline concerned

In the United States, FAA approved seats are often the only seats a child may sit in. Your car seat may be NTSB approved for driving, but not for flying. You may wish to check your air carrier and make sure you check the car seat for FAA approval.