The goings-on in front of six young kids at 647 NW Third Ct. in Hallandale Beach — the dope smoking and drinking, the yelling, cursing and threats — was so disturbing to an observer that she called the state’s child abuse hotline.

“It is just terrible,’’ the woman told the hotline operator just after the New Year. “Horrible.’’

Four days later, the 21-year-old mother of some of those children, Brittney Sierra, would be arrested for child neglect, which could be the least of her problems.

Investigators dug up the remains of an infant in a nearby yard on Friday. They might be Sierra’s son, Dontrell Melvin. If they are, Sierra, 21, could be charged with murder along with the boy’s father, Calvin Melvin, 27.

The scene at the house Sierra and her children shared with her mother, Renee Menendez and her children, shouldn’t have been a surprise to child-welfare workers, considering how many times they’d been there.

Department records indicate 30 contacts with Menendez, a KFC manager in her 40s who was raising four children ranging in age from 8 to 11 — Sierra’s half siblings.

Like her daughter Sierra, Menendez has now lost all of her children. Department of Children and Family workers moved them to a state home over the weekend.

They also took Sierra’s two remaining sons, a boy named Jaydyn, and an infant born in August.

Records released by DCF on Monday include a transcript of the call describing the wretched conditions at the house — Menendez’s boyfriend called the children “mother-----s’’ and “bitches,’’ the caller said, and screamed that he would “knock them around.’’ The agency also released transcripts of three prior calls from people concerned for the Menendez kids.

But DCF personnel reported over and over that there was no need to step in.

Though up to seven youngsters were supposed to live at the house, a child welfare investigator who visited the house in September 2012 saw only the older children.

The investigator never saw Dontrell, born in February 2011, or the newborn infant.

Nor did anyone from DCF see Dontrell a month later, on Oct. 16, when the hotline received another, far more chilling report: “Dontrell has not been seen since 2011.”

The case is eerily similar to another scandal that rocked DCF two years ago. That’s when a road ranger in Palm Beach County found a red pickup truck awash in toxic chemicals pulled over the side of Interstate 95.

The discovery, police believe, came days after a DCF investigator visited the home of Nubia and Victor Barahona — and failed to see the children.

James Sewell, a former Florida Department of Law Enforcement agent and DCF consultant who sat on panels that reviewed two child deaths, including Nubia, said the department seems fated to make the same mistakes over and over.

“I’m tired of dead baby cases,” said Sewell.

“There is no reason, absolutely no reason,” he said, for an investigator to visit the home but leave without looking at all the children the department knows to be living there.

But Monday night, DCF Secretary David Wilkins said the October call reporting Dontrell missing was clearly a missing child report, “which this agency does not have the legal authority to investigate.”