The police found about 10 grams of marijuana, or about a third of an ounce, when they searched Penelope Harris’s apartment in the Bronx last year. The amount was below the legal threshold for even a misdemeanor, and prosecutors declined to charge her. But Ms. Harris, a mother whose son and niece were home when she was briefly in custody, could hardly rest easy.

The police had reported her arrest to the state’s child welfare hot line, and city caseworkers quickly arrived and took the children away.

Her son, then 10, spent more than a week in foster care. Her niece, who was 8 and living with her as a foster child, was placed in another home and not returned by the foster care agency for more than a year. Ms. Harris, 31, had to weather a lengthy child neglect inquiry, though she had no criminal record and had never before been investigated by the child welfare authorities, Ms. Harris and her lawyer said.

“I felt like less of a parent, like I had failed my children,” Ms. Harris said. “It tore me up.”

Hundreds of New Yorkers who have been caught with small amounts of marijuana, or who have simply admitted to using it, have become ensnared in civil child neglect cases in recent years, though they did not face even the least of criminal charges, according to city records and defense lawyers. A small number of parents in these cases have even lost custody of their children.