JetBlue now guarantees that children ages 13 and younger can sit next to an accompanying adult without paying an extra cost.
The airline has enabled the family seating guarantee through a technology upgrade that identifies reservations with adults and children traveling together who did not secure seating assignments at booking.
"For no additional fee, the process will ensure a traveling child is assigned a seat next to at least one accompanying adult when seats are available," the airline said Wednesday.
The policy applies to all JetBlue flyers, including those who purchase a Blue Basic fare, which typically doesn't offer free assignments prior to check-in.
The airline said that for bookings made within 24 hours of planned travel, seats might need to be assigned manually by a crewmember.
JetBlue completed the technology update under pressure from the Department of Transportation, which is drafting regulations that will require airlines to seat children 13 and under with an accompanying adult free of extra fees.
The House also included a similar provision in its five-year FAA reauthorization bill, which it passed in July.
Still, most airlines have not yet fallen in line voluntarily. Only American, Alaska, Frontier and now JetBlue have, according to the DOT's Airline Customer Service Dashboard, but United last winter did a technology upgrade making it easier for children 11 and under to sit next to an accompanying adult without extra charges.