In the UK a 2018 CAA study found that this splitting up of families may be happening deliberately. From 4200 journeys, Ryanair was found to split up 35 per cent of travel companions who had not paid for seat allocation.
Then business secretary Greg Clark called out the practice as "evil" at a 2018 UK Conservative Party conference, saying that passengers were being "nudged to pay more to sit together".
Pre-pandemic the CAA found reported that seating allocation charges were worth $800m a year to airlines flying out of the UK.
In a DOT review from 2018, the department said that one in every 200 complaints was related to family seating on planes.
What are the rules for flying with children?
Most airlines will have seating allocation policies for families flying together. However this varies airline by airline and depends on the flight destination and their air laws.
For passengers flying to Canada, for example, accompanying passengers over the age of 16 must be allocated seating next to children.
Many airlines interpret the condition of seating children, "adjacent to the seat of an accompanying family member" is often read as a seat in the row in front or behind. Something that parents may take issue with.
There are many complexities of allocating seats to children, especially internationally. Minors cannot be seated in emergency exits for example. While airlines are required to seat young children with adult carers, passengers flying into New Zealand are able to leave children over five in a separate cabin class while parents fly business.
"If you don't select your seats in advance, we'll do our best to find adjoining seats for your family on the same reservation in the cabin you booked a few days before your flight departs," reads the United website. "However, available seats may be limited at that time and your family or group may be split. While we strive to seat your family together, seat selections are not guaranteed and may be changed, including in the event of an aircraft substitution."