An NYPD officer and his fiancée were arrested Friday morning on Long Island in the death of the transit cop’s 8-year-old son, who was initially said to have died after falling in a driveway, police and sources said.

Michael Valva, 40, a 15-year veteran, and his fiancée, Angela Pollina, 42, both of 11 Bittersweet Lane in Center Moriches, were charged with second-degree murder, Suffolk County police said. The cop has been suspended without pay, sources said.

Police responded about 9:40 a.m. Jan. 17 for a report of a child who fell in the driveway of the Suffolk County home.

Thomas Valva was rushed to Long Island Community Hospital in Patchogue, where he was pronounced dead. The Suffolk County medical examiner is performing an autopsy.

“Thank God the justice is being served,” the boy’s tearful mother, Justyna Zubko-Valva, 36, told Newsday on Friday, adding that police had not yet contacted her.

Zubko-Valva, a corrections officer on Rikers Island who lives in Valley Stream, said her son had a mild form of autism and was a second-grader at East Moriches Elementary School.

In an interview after her son’s death, Zubko-Valva described her son as a “loving, happy boy, always looking for adventure, loving playing outside.”

Tommy, she said, “had such an amazing good heart. Everybody who knew him, they instantly fell in love with him. He was just a joy of everybody’s life.”

The mother had lost custody and had not seen Thomas and his two brothers for two years, according to CBS 880.

“Unfortunately, I saw him now just looking at his body in a medical examiner’s office,” she told the outlet Tuesday. “And I do pray that one day we’re just going to meet again in heaven. I know he’s in heaven. I know he’s my angel and I’m going to miss him so much.”

The father previously released a statement through a lawyer, saying, “As with any tragedy, our office and Mr. Valva are shocked and saddened to learn of the horrible accident that took the life of young Thomas Valva,” according to WCBS.

“We mourn his death with family and friends. Since Thomas’ parents are in a divorce we are unable to make any further comments at this time.”

The grieving mom said Thomas’ death “is not going to go in vain.”

“I miss him so much,” she told WCBS. “Thomas was this little 8-year-old boy, my baby, who always was joyful, always stands for the truth, play his cars, read books, loved songs.”

A GoFundMe page has been set up to help with funeral expenses.

“It is with great sadness that I must bury my 8 year old son. He passed recently due to a tragic accident. At this time I am not able to handle these unforeseen costs on my own and if there is any help anyone can provide we would greatly appreciate it,” the fundraiser says.

A spokeswoman for the Suffolk County DA’s Office said Valva and Pollina are set to be arraigned later Friday.