The phrase "lap child" refers to a small child who sits on an adult's lap during a flight or car ride instead of having their own seat. Full definition
Also, many charge for lap children on international flights. (entrepreneur.com)
Without a ticket, the baby has to be held on your lap throughout the flight, and some airlines charge a «lap child fee,» especially for international flights. (asherfergusson.com)
She's only under - two for a few more weeks so we're taking full advantage of her fleeting lap child status. (ahintofhoney.com)