When Jennifer Huff checked in for her US Airways flight at Reagan National Airport recently, she was told that she would be in seat 4C. Then came the surprise: Her 2-year-old daughter, Alexa, had been assigned to seat 11C, seven rows behind.

“What if she gets sick? What if she’s scared?” Huff, 34, of Alexandria, Va., said at the thought of sitting apart from her toddler on the three-hour flight to Pensacola. “Someone is going to have to change that diaper.”

Huff persuaded the ticket counter attendant to move her within one row of her daughter. It wasn’t until she and Alexa reached the gate that another airline employee clinched them adjacent seats.

The stressful quest for seats is one that parents have begun to encounter with increasing frequency in recent years as more airlines have instituted extra charges for “preferred” or “premium” coach seats, leaving fewer free seats together. The issue has drawn attention on Capitol Hill.

For people traveling alone or on business, the extra fees — most range from $5 to $100 one way for domestic flights — can amount to a relatively minor expense for an aisle seat, extra legroom or an unobstructed view.

But for families, particularly those with multiple children, such fees can add hundreds of dollars to the hefty costs of group travel. On some flights, 30 percent to 40 percent of coach seats are now reserved for those willing to pay extra, airline experts say. Passengers not willing to pay up often are left with middle seats, rows apart.

United Airlines started the trend in the United States in 2005, when it made its “Economy Plus” seats — with more legroom — available not just to frequent fliers but to any passenger willing to pay extra. In 2008, US Airways began selling window and aisle seats toward the front as “Choice Seats,” and in late 2011 Delta began selling “Preferred Seating.” American Airlines unveiled its “Main Cabin Extra” seating, with more legroom and early boarding, in March.

Loree Tillman, 44, of Houston said she is often assigned a seat away from her 9-year-old son, Tyler, because she can’t afford premium seats. For Tillman, it’s not just an inconvenience — it’s also a matter of safety and peace of mind.

“The person sitting next to him hasn’t had a background check,” Tillman said recently as she and Tyler waited at Baltimore-Washington International Marshall Airport for their flight home. “I don’t know if he’s a pervert.”

Tillman held up their US Airways boarding passes. Her son had been assigned to 30B, she to 29E. If the flight attendants wouldn’t help, she said, she would persuade the person seated next to her son to switch.

“I tell people, ‘He talks a lot — it’s a long flight,’ ” Tillman said. “I say, ‘Do you like Pokemon? Do you like Nintendo? Because he’s going to tell you all about it.’ ”

Parents interviewed recently at Washington area airports said fellow passengers eventually agree to switch seats or are persuaded to do so by flight attendants. But the uncertainty heightens the anxiety and stress of flying with children.

Airline cost-cutting has led to fewer (and fuller) flights, and finding a spare seat to facilitate a switch has become more difficult. Meanwhile, passengers who are traveling without children and who paid extra for a particular seat can find themselves with two equally unappealing options: Relinquish a premium seat or spend hours babysitting someone else’s cooped-up child.