Mayor Bill de Blasio promised to fight against New York’s “tale of two cities” — but his administration just keeps causing the worst of times for the Big Apple’s most vulnerable.

A day after admitting there were nearly three times as many lead-tainted NYCHA units as previously known, a judge on Tuesday shot down an administration decision to give a contract to care for abused kids to a cut-rate company that was found to be unqualified.

City Hall saved a mere $1.8 million — out of a $3 billion Administration for Children’s Services budget — by selecting Gotham Per Diem for the three-year, $8.5 million child care job.

Among those that the provider was contracted to help were LGBTQ kids and underage victims of sex trafficking.

Gotham got the contract earlier this year by underbidding another company, Tempositions Health Care Inc., which had done the job providing staff to temporary housing facilities for kids since 2014.

Tempositions, however, sued to prevent Gotham from actually taking charge of the kids by going to Manhattan Supreme Court and filing a claim that its competitor was not legally qualified to take on the job.

Though Gotham argued that it did have the relevant experience, Justice Eileen Rakower found it didn’t meet the agency’s requirements of having worked for three previous organizations where the staff-to-child ratio was three-to-one.

“Gotham only showed that it served two clients with the approximate ratio of one staff member to every three children, but it was required to make a minimum showing that it served three clients with this ratio,” the judge ruled.

“Accordingly, ACS failed to comply with its own rules,” she added as she annulled the deal. She, however, left the decision about what would happen to the contract up to ACS.

A City Law Department spokesman said, “We’re reviewing the court’s ruling.” They added that kids will continue to receive necessary services as Tempositions is still providing the care until the contract issue is hashed out.

Aaron Zerykier, lawyer for Gotham, said the company “is exceptionally qualified.”