Children under 2 years (24 months), are not required to have their own seat. You must, however, inform Alaska Airlines that you will be traveling with an infant on your lap.

On Alaska Airlines, children under the age of two (0–23 months) are eligible to fly for free as "lap infants" on domestic flights within the United States. To qualify, the child must be held in the lap of a ticketed adult (18 years or older) for the duration of the flight. While the base fare is free, the adult must still notify the airline to have the infant added to the reservation. For international flights (such as to Mexico, Canada, or Belize), the infant still doesn't require a seat, but the adult must pay the associated international taxes and fees, which typically range from $18 to $54 depending on the destination. Once a child reaches their second birthday, FAA regulations strictly require them to have their own purchased seat and a confirmed ticket at the standard adult fare. Alaska Airlines allows one lap infant per ticketed adult; if a single adult is traveling with two infants under two, one must be in a purchased seat. It is also worth noting that if an empty seat is available on a flight, Alaska often allows a lap infant to occupy it for no extra charge, though this is never guaranteed and depends on the flight's final load.