Mum Angela Persaud, from Long Island, New York, says she noticed burns on her son's fingers and legs when she was giving him a bath.
She says she reviewed CCTV footage from her Nanny Cam and saw 21-year-old Nxumalo allegedly burning Xander with the curling iron.
"You see her with the curling iron, and she touches it to make sure that it’s hot," Persaud told WABC.
"And then when he jumps because it burns him, she continues to touch his foot again and then goes for his hand."
Persaud called police after reviewing the footage.
"You can tell that she’s telling him, ‘That’s what happens when you don’t listen,’ and then she hugs him," Persaud told WABC.
"So she’s playing these warped mind games with my child, and it’s the most disgusting thing I’ve ever witnessed in my life."
Nxumalo, from Harlem, was arrested and later admitted that she had been frustrated with the child and was trying to discipline him, authorities said.
She reportedly told police: "It was a long week and I was tired. The child wouldn’t listen so I wanted to discipline him with the iron, so I burned him with it."
Nxumalo, who had worked for the family for a matter of weeks, has been charged with two counts of assault, criminal possession of a weapon and endangering the welfare of a child.