A 12-year-old boy and 8-year-old girl concocted an elaborate story about a stalker — causing an innocent man to be grilled by cops before their prank was finally revealed.

The Brooklyn kids, whose father is a special-victims detective, even penned a fake note to frame a passer-by who claimed he once lived in their home, according to law enforcement sources.

“Watch out. I am watching you!! Your daughter is cute,” the letter read.

The pranksters also told police the man had followed the girl as she walked with her friend from her Crown Heights home to the school bus pickup spot near Eastern Parkway and Brooklyn Avenue.

They said the same man showed up at the family’s Saint John’s Place home around 7 p.m. Friday, where the girl said she spotted him on the steps outside the building “ducking down and taking pictures of the interior of the house.”

The girl said when she confronted him, he asked her where in the house she sleeps and who lives with her — but fled when she refused to answer.

The boy said the man returned to the home the next day at 11:15 a.m. and asked him questions about his parents and what bedroom he sleeps in.

The boy didn’t answer him and walked into the house.

The cops went public that they had a stalker on their hands and the suspect was apprehended by police — but then the kids’ story began to fall apart.

The individual had walked by the apartment, but did not appear to do anything threatening or harmful.

Surveillance video caught the man in front of the home, but sources said he claimed he once lived at the address and just wanted to drive by his old home.

The kids eventually admitted making up the story and writing the letter themselves and placing it in their mailbox.

Sources said the parents were away on vacation and the youngsters were unhappy with their strict babysitter.

After being grilled by cops, the man was released Monday night without any charges filed.