The 2-year-old migrant girl who crossed the border alone Sunday with only a note is an example of a child who could fall prey to sex traffickers thanks to overextended, underfunded US agencies, officials say.

The country’s broken immigration system is notorious for allowing thousands of kids to slip through the cracks, many of whom fall into nightmarish lives of abuse and exploitation, they said.

“This is such a stain on our nation,” said an exasperated Tara Lee Rodas, a whistleblower with the federal Department of Health and Human Services, to The Post on Monday.

Texas Department of Public Safety Lt. Christopher Olivarez said, “It’s very concerning when we come across any type of child that’s coming across the border alone, especially with a piece of paper, because we don’t know exactly what happens to them on their journey and where they ended up afterwards.”

The unidentified child who was found Sunday had been among a group of around 60 unaccompanied children from El Salvador who had just illegally crossed over the US-Mexico border. The tot, clad in a bright pink jacket, was holding a small sheet of paper with nothing but a first name and phone number written on it.

In heartbreaking video shared by the Texas DPS, the tiny child told cops that she was 2 years old, traveling alone and was going to meet with “her mom and dad.”

Authorities told The Post she would most likely be released to the HHS, which would make every effort to track down a US guardian. If a US guardian for her can’t be found, and the child’s relatives can’t be tracked down in El Salvador, she will go into government care in the US, officials said.

But once she leaves HHS, there aren’t enough resources to see her case and similar heartbreaking ones through beyond that, they said.

Good-faith efforts to prevent such children’s exploitation can easily be thwarted by sex and labor traffickers, who often terrorize their helpless victims to ensure compliance, authorities said.

Olivarez said many children are threatened by traffickers not to divulge to law enforcement at the border that they are in danger, making detecting the abuse a nearly impossible task.

Rodas said her federal agency does little besides a phone call to try to verify a child is where they say they are supposed to end up.

“We’re picking up the phone. … We’re not seeing this person face to face,” she said of the alleged responsible caregiver on the other end.

“We’re not doing DNA testing, and we’re turning small children over to someone in the US that the child gets a piece of paper and says, ‘This is where I’m going.’  I mean nobody’s questioning that,” Rodas said.

She said she believes that many of the roughly 320,000 migrant kids in the US who have lost contact with the feds after their release to sponsors here have ended up in the clutches of sex and labor traffickers.

House Homeland Security Chairman Mark Green (R-Tenn.) called the situation “unconscionable.

“In a hearing just last week, a whistleblower and former HHS employee revealed the Biden-Harris administration has routinely failed to properly screen and vet sponsors of these children in order to expedite processing amid the border crisis,” Green told The Post in a statement.

“Putting optics over safety has led to countless children in danger or unaccounted for. This refusal to protect vulnerable alien children from abuse, exploitation, and human trafficking will be one of the defining failures of the Biden-Harris administration.”