“The U.S. Department of Transportation (DOT) encourages all airlines to guarantee that young children are seated adjacent to an accompanying adult without charging any additional fee.” https://www.transportation.gov/airconsumer/airline-family-se...

There is not coverage beyond one adult already in the US. With an additional adult and one child, the airlines already adds in fees. It’s also non-transparent when booking that they have made sure the easy path is the charged path, especially now that airlines make you pay to guarantee being seated together prior to flight checkin 24 hours in advance of takeoff.