Mayor Bill de Blasio said: "This is racism. Plain and simple."
"It would be horrific at any age, but it's especially offensive that it happened to a child," he said in a tweet.
Prosecutors in Manhattan are now investigating the incident.
In a statement, the parents of Keyon Harrold Jr and civil rights lawyer Ben Crump called on the Manhattan district attorney to bring assault and battery charges against the woman "to send the message that hateful, racially motivated behaviour is unacceptable".
They also called for a civil rights investigation into the Arlo SoHo Hotel for its "implicit" bias in the way it treated Kenyon.