The incident set off a firestorm on Twitter early Sunday after Shannon Watts, founder of the gun control advocacy group Moms Demand Action, posted about the girls not being allowed to board.
According to Shannon Watts, who was at a gate at Denver International Airport and had started tweeting about the incident, she heard the agent explain to the young travelers they couldn't get on the plane wearing Spandex.
Per United's policies in their Contract of Carriage, rule 21 states that "pass riders" flying under company benefit travel can be held from boarding a flight if they are "barefoot or not properly clothed". Celebrities such as William Shatner, Sarah Silverman and Christine Teigen chimed in - all critical of the airline.
The young girl, who was wearing gray leggings, not too tight, said Watts, pulled a dress from her backpack and put it over her leggings and was allowed to board, Watts said.
She added that the member of staff at the gate insisted she did not set the rules but was merely enforcing them. Others say the airline is wrong to police the attire of 10-year-olds.
- Patricia Arquette (@PattyArquette) March 26, 2017 @united @PattyArquette Friendly Advice, United: No one is almost as offended by a little girl in spandex as they are by this situation.
Coleman leg break mars Republic of Ireland's draw with Wales
Ireland slip to second in the Group D standings, level on points with Serbia at the top but behind on goal difference. Not only that, but Wales would have been cut seven points adrift and virtually out of contention.
The row began when a fellow passenger took to Twitter in bewilderment on Sunday morning after watching the agent tell the girl, and two others, that they were inappropriately dressed.
"United shall have the right to refuse passengers who are not properly clothed via our Contract of Carriage".
'United Airlines is defending their decision to prevent pre-teen girls wearing leggings from boarding a flight due to their attire, ' another wrote alongside a string of thumbs-down emojis.
"I have flown united before with literally no trousers on".
"I have five kids: four of them are women".
After the Daily Dot's Gavia Baker Law tweeted at the airline, United said it was investigating. Amidst the outpouring of criticism, some said they would boycott the airline. The people who are mad just think it's an incredibly stupid and sexist policy.