Celia Hahn didn't think much of it when she didn't receive a seat assignment while booking flights for her family last year.

On her Delta Air Lines reservation she entered the ages of her twin boys, then 8, and assumed they would be seated next to her. Instead, because of the type of tickets she booked, they were scattered throughout the plane from Atlanta to Minneapolis-St. Paul. They weren't allowed to swap seats with generous strangers until the flight took off.

"We took off and my kids were in different parts of the plane crying," she said. "People were willing to give up their seats, but it was stressful and more so than it needs to be."

Air-travel experts said holiday periods like this week are ripe for such problems.

The U.S. Department of Transportation recently decided against implementing a policy that would require airlines to seat minor children next to an accompanying adult. This leaves it up to the discretion of the airlines as to how it handles family bookings when seat assignments are not included in the price of their tickets.

Each U.S. airline's policy is different, but parents often realize once it's too late they will be separated from their children.

Changes in the airline industry over the past several years have created prime conditions for such situations. First, airlines started packing planes more full, leaving less wiggle room for swapping seats. Second, airline number crunchers realized not every airplane seat is worth the same amount of money. They assigned incremental values to each seat based on a variety of desirability factors — like legroom, exit rows, amount of recline, distance from a bathroom and the age-old window, middle or aisle seat preference.

But it wasn't until the advent of "basic economy" airfares, the cheapest seats, which don't allow customers to pick their seats without paying a fee, that family seating really became complicated.

"These problems are just going to occur more frequently," said Bob Mann, a Port Washington, N.Y.-based aviation analyst. "It's a real issue and I don't think the DOT has done itself any favor by taking a pass on it. The industry also needs to take a step back and decide if they want a black eye on this or solve it."

In 2016, Congress instructed the DOT to review and, if appropriate, establish a policy for airlines to seat passengers under 13 next to an accompanying family member or adult who was older than 14.

Several years later, DOT concluded it wasn't a big enough problem to mandate a solution. Instead, the agency created a webpage that has links to every U.S. airline's family seating policy.

According to documents obtained through a public records request by Consumer Reports, the DOT received 136 complaints between March 2016 and November 2018 related to airlines separating children from their parents or guardians.

"Numerous complaints involve airlines knowingly assigning seats apart from family for children as young as 2 years old," according to a Consumer Reports review of the complaints. Others cite emotional trauma of children sitting alone, including those with autism or seizure conditions. Many parents with bad experiences mentioned anxiety about their children not being prioritized or cared for in the case of an emergency, and fear of sexual assault.

Airlines said they do everything they can to make it clear to passengers during booking what is and is not included with their airfare, especially for basic economy. They also point out passengers can always buy the higher-fare class that includes seat selection.

After Hahn booked her family's flights, she followed instructions and checked in 24 hours before their scheduled departure. Only middle seats scattered throughout the plane remained open.

She called Delta and was told she could get seats together at the ticket counter. After arriving early for their outbound flights at Minneapolis-St. Paul International Airport, she was able to get seats together for three of the four legs of their round-trip to Belize. The last leg home — from Atlanta to MSP — looked full and the agent said they would have to wait until the flight took off to rearrange their seats with strangers.

"I was pretty irate. How can you seat my minor child next to a stranger?" Hahn said. Since then, she has reluctantly booked the more expensive tickets, but said not everyone can afford to do that or understands the consequences of booking the lower fares.

A Delta spokesman said, "Regardless of the type of ticket purchased, Delta works with customers on a case-by-case basis to ensure their travel needs are met. When customers have seating questions, we encourage them to reach out to us as soon as possible to allow for the opportunity to address their concerns."

The problem, said Rainer Jenss, president of Family Travel Association, is that the current laissez-faire approach doesn't differentiate between what is a luxury perk and what is a necessity.

"Flying next to a spouse or significant other is a premium; flying next to your 4-year-old is not the same thing," Jenss said. "The policy is currently discriminating against parents by forcing them to pay for or do something that is necessary, not a convenience."

Some countries, like Canada, have come to the same conclusion.

On Dec. 15, Canada's new Air Passenger Protection Regulations went into effect, including a provision that requires airlines help to seat children under age 14 near their parent, guardian or tutor "at no extra cost and at the earliest opportunity." Children under 5 must be given an adjacent seat, those between 5 and 11 must be in the same row and separated by no more than one seat, and those 12 or 13 cannot be separated by more than a row from their accompanying adult.

Efforts to legislate a solution by members of the U.S. Congress have continued, including a bipartisan bill, called the Fly Together Act, introduced earlier this month by Reps. Ann Wagner, R-Mo., and Anthony Brown, D-Md.

U.S. Sen. Chuck Schumer, D-N.Y., has also continued to press DOT to take greater action on the issue.

There are technological solutions the airlines could implement. "The technology is not that difficult, but it requires the airlines make some decisions," said Mann.

Jenss suggests airlines focus their big data and algorithms, which they are already using for other purposes, on better forecasting how many families will be flying on a given route at that exact time of year.

"They should be able to block off the last X-amount of rows — not premium seats — for X-amount of families. They just haven't made it a priority," Jenss said. "But for now, the airlines are relying on the generosity of strangers being willing to move, and sadly, that doesn't always happen."