A stranger walked off with two 4-year-olds she found playing in the back yard of a Brooklyn apartment building Saturday afternoon, setting off a frantic, two-hour search by police and relatives.

Keren Cohen, 46, had struck up a conversation with the girl and boy, who are the niece and son of tenants of the 51st Street building, cops said.

She claimed they had asked to be taken to a nearby park, and she agreed, cops said.

Police found the kids walking home unharmed.

Cohen, who was still in the park, was charged with criminal trespass and endangering. Her motive was unclear, police said.