The issue has caught the attention of at least one member of Congress. In May, Sen. Charles Schumer, D-N.Y., asked U.S. Transportation Secretary Ray LaHood to prevent airlines from charging extra for families that need consecutive seats. Schumer also asked the airlines to voluntarily waive such fees for passengers with children.

LaHood responded in July that the Transportation Department has no authority to regulate such fees. LaHood said he had spoken with airline executives and “urged them not to charge fees that would negatively impact families.”

“Nevertheless,” LaHood wrote, “more needs to be done, and I will continue to raise this issue with the airline industry.”

Jean Medina, a spokeswoman for the airline industry group Airlines for America, said seat fees are part of the industry’s changing business model.

With “razor-thin profit margins” — profits averaged 77 cents per passenger per flight last year — airlines must charge for such things as premium seating and early boarding to keep overall ticket prices lower, she said.

Families are most likely to be separated when their seats are assigned at the airport, airline officials say. Airline websites that generally sell out seats available at no extra charge often require people to get them at check-in. Passengers who book through some non-airline travel websites also can’t get seats confirmed until check-in.

Airline officials said they use seats that remain unassigned until 24 hours before a flight to re-seat family members together. When that fails, they said, gate agents and flight attendants step in.

“I’ve worked for American Airlines for 20 years, and I’ve never heard of an instance when we haven’t been able to arrange for a parent to sit with their child,” said Mary Frances Fagan, an airline spokeswoman.

In a recent website posting — titled “Do airlines hate families?” — the traveler advocacy group Consumer Travel Alliance called on airlines to waive fees for children 6 and younger to be seated with a parent. The need to switch seats at the last minute to keep families together complicates the check-in and boarding process for all passengers, said the group’s director, Charlie Leocha.

“There’s this period of uncertainty,” Leocha said. “People are having to barter for seats at the airport. They’re playing seat roulette, which is just not fair to customers.”

Charlie Hobart, a United Airlines spokesman, said families end up separated most often when the airline changes the type of aircraft assigned to the flight. The computer is programmed to keep families together when it reassigns seats, he said. When that doesn’t work, he said, gate agents get involved.

Jamie Pearson, publisher of the Travel Savvy Mom website, said she is resigned to the added costs of traveling with children. Airlines cater to top-paying business travelers and frequent fliers more than to families that fly together once or twice a year, Pearson said. In November, she said, she paid $130 extra, round-trip, for seats together when she and her then-12-year-old daughter flew American Airlines between San Francisco and Dallas. The checked bag fees added another $100, round-trip.

“What can you do?” Pearson said. “The airlines are in trouble. They’re shaking us down. . . . I think whining about it is sort of silly. It’s the real world. Airlines aren’t nonprofits.”