WASHINGTON — At the age of 6, a child is full of imagination and may not distinguish reality from fantasy. She is beginning to read and can’t grasp nuances in written communication. She also doesn’t understand privacy.

Citing those reasons and more, dozens of pediatric and mental health experts are calling on Facebook to kill a messaging service the company introduced last month for children as young as 6.

In a letter to the company, they said the service, Messenger Kids, which pushes the company’s user base well below its previous minimum age of 13, preys on a vulnerable group developmentally unprepared to be on the social network.

The letter was organized by the Campaign for a Commercial-Free Childhood, an advocacy group that has successfully pushed companies to abandon marketing like a Pokemon Go app that sent children to fast food and other stores, and McDonald’s advertising on the envelopes of report cards in Florida.