A teenage baby-sitter, her Ronkonkoma mother, and the girl’s boyfriend were arrested after the trio briefly left an infant on bench before leaving the child with security personnel at an Islandia casino Tuesday evening, police said.

Each was charged with endangering the welfare of a child after Suffolk police said they abandoned the 9-month-old baby boy in a dispute with the baby’s mother.

Police said the sitter is 14, but she claimed to be 18 when she was hired to baby-sit the child off the website Sittercity.com.

Suffolk Police Commissioner Timothy Sini said the trio should have taken “appropriate action” and called police.

“Clearly these individuals did not do the right thing and that’s why they’re being criminally charged with endangering the welfare of a child,” Sini said.

The incident unfolded in two locations Tuesday evening, police said. An initial call focused on a residence on Johnson Avenue in Ronkonkoma, identified by police as the home of the baby-sitter and her mother, Dejuana Stewart.

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Police said that the trio — Stewart, 44, her daughter and the daughter’s 16-year-old boyfriend from Brooklyn, whose names were not released because they’re juveniles — called them at 6:40 p.m. to report the child’s mother had failed to arrive at the house to pick up the infant at the designated time.

Police said when Fourth Precinct officers arrived at the home, “the caregivers stated that they had resolved the issue and had arranged to meet the baby’s mother.”The trio then took the baby to Jake’s 58 Hotel & Casino on Express Drive North in Islandia — the location where they had arranged for the mother to pick up the child — police said, but “decided to leave the child at the casino when the baby’s mother failed to arrive.”

It was then, police said, that the Brooklyn teen left the child — who was in a carrier, on a bench outside the casino — and walked toward his car. Police said the teen returned “in a matter of minutes” and took the baby inside — telling casino security personnel he had found the child on a bench.The trio then left, police said.

During a review of security surveillance footage, police said, officers recognized the boyfriend from the prior call on Johnson Avenue and arrested him.

With Carl MacGowan and Nicole Fuller