(CNN) What used to be an "extraordinarily rare occurrence" has already happened twice this month -- the death of a child in the custody of US border officials.

Now the head of Customs and Border Protection is reiterating calls for Congress to provide more funding to handle the influx of migrants crossing the border.

"I've explained to Congress ... that what we're seeing with these flows, of huge numbers of families with lots of children, young children, as well as unaccompanied minors coming into Border Patrol custody after crossing the border unlawfully, that our stations are not built for that group that's crossing today," CBP Commissioner Kevin K. McAleenan told "CBS This Morning" on Wednesday.

"They were built 30 to 40 years ago for single adult males, and we need a different approach. We need help from Congress. We need to budget for medical care and mental health care for children in our facilities."

In the past three weeks, two Guatemalan children have died after they were detained with their fathers after crossing the border.

On December 8, Jakelin Caal Maquin, 7, died in a hospital two days after she was taken to a Border Patrol station.

And on Christmas Eve, an 8-year-old boy died after he was taken to a hospital, released, then returned to the hospital . CBP has not named the boy, but US Rep. Joaquin Castro identified him as Felipe Alonzo-Gomez.

The CBP chief called the young boy's death "a tragic loss."

"On behalf of US Customs and Border Protection, our deepest sympathies go out to the family," McAleenan said Tuesday.

Homeland Security Secretary Kirstjen Nielsen on Wednesday called the death of a child in government custody "deeply concerning and heartbreaking," and reiterated her call for "parents to not place their children at risk by taking a dangerous journey north."

Criticism grows

"We're not designed to hold all these people, a complete overhaul needs to happen," a US Customs and Border Patrol official who works on the southern border and has direct knowledge of the facilities like the ones where Felipe Alonzo-Gomez was being held told CNN Wednesday night.

"We're overcrowded, understaffed. We don't have the manpower to deal with this crisis. It's a nosebleed that doesn't stop. You have agents trying to do the right thing. Give people their due process," the official notes. When asked about DHS Secretary Nielsen's handling of the Guatemalan child's death this official replied, "What else can she do?"

"No one ever thinks you'll lose a life. You give them food and water. If they complain, you flag it. It's sad. It's heartbreaking. That hits home."

"I think there is a major failure on behalf of the agency, but also a major failure in Congress. Instead they're protesting. Where were you when we needed you? The ones that are getting caught in the middle are the guys on the front lines. Those are the guys that are getting hurt."

Speaker-designate Nancy Pelosi said Congress will investigate the two deaths to "ensure that no other child is left to such a fate."

The tragic death of an innocent child on Christmas Day breaks the hearts of all. Democrats call on Homeland Security's Inspector General to immediately open an investigation into Felipe Alzono-Gomez's death. https://t.co/on2E63hPhX pic.twitter.com/pvNCtMJt9g — Nancy Pelosi (@NancyPelosi) December 26, 2018

Policy changes

Homeland Security and CBP officials announced a series of new procedures following the two children's deaths:

• Border Patrol conducted secondary medical checks on all children in CBP care and custody, with a focus on children under age 10.

"Moving forward, all children will receive a more thorough hands-on assessment at the earliest possible time post-apprehension -- whether or not the accompanying adult has asked for one," Nielsen said in a statement Wednesday.

• Border Patrol is working with Immigration and Customs Enforcement on surge options for transportation to Family Residential Centers and supervised release, CBP said. The agency also is reviewing other custody options to relieve capacity issues in the El Paso, Texas, sector, such as working with nongovernmental organizations or local partners for temporary housing.

• Nielsen said she has asked that Centers for Disease Control experts investigate the increase in sick migrant children in order to identify steps hospitals along the border can take to treat them.

• The US Coast Guard Medical Corps has been asked to assess CBP's medical programs and recommend improvements. The Defense Department has been asked to provide additional medical professionals.

• Finally, CBP is reviewing its policies, with a focus on the care and custody of children under 10 -- both at intake and beyond 24 hours in custody, the agency said.

What happened to the 8-year-old boy

CBP released a timeline of the days before the boy's death . On Monday, the boy was taken to a hospital, diagnosed with a cold, released with medication, then taken back to the hospital before dying about 12 minutes before Christmas.

The cause of the boy's death is not clear. But an autopsy is scheduled for Wednesday in Albuquerque, according to the University of New Mexico Office of the Medical Investigator

Castro called for a congressional investigation into the boy's death.

"While the CBP notified Congress within 24 hours as mandated by law, we must ensure that we treat migrants and asylum-seekers with human dignity and provide the necessary medical care to anyone in the custody of the United States government," the congressman said Tuesday.

Asylum A protected status that allows people fleeing persecution to live legally in another country.

Many people wonder why asylum-seekers cross illegally rather than at a legal point of entry. But asylum-seekers are often at the mercy of wherever smugglers drop them off, which can be far from a legal point of entry.

And according to US law, even some who enter illegally can seek defensive asylum

Dozens of hospital trips every day

The Department of Homeland Security said it encounters hundreds of thousands of migrants a year. Before this month, there were six deaths in CBP custody in 2018, and none of those were children, DHS officials said.

Before Jakelin's death, no child had died in CBP custody in more than a decade, a Department of Homeland Security official said.

In the last two months alone, the Border Patrol has apprehended 139,817 undocumented migrants on the Southwest Border, compared to 74,946 during the same time in fiscal 2018, Nielsen said.

"It is now clear that migrants, particularly children, are increasingly facing medical challenges and harboring illness caused by their long and dangerous journey," said Nielsen, who announced that she will travel to the border later this week to review medical screenings and Border Patrol stations.

Still, the death of a child in custody is "an extraordinarily rare occurrence," McAleenan told CBS.

But now, as more children cross the border, "We're doing dozens of hospital trips every single day with children that have fevers or manifest other medical conditions," McAleenan said Wednesday.

"We've asked for help. We've got two Coast Guard teams deploying today to support our Border Patrol agents in checking the welfare of children in our custody."

President Donald Trump has reiterated calls for a border wall and said the current government shutdown won't end until a wall, fence or other border barrier is funded.

When asked whether the government shutdown for a border wall was worth it, McAleenan told CBS.

"We need border security investments, absolutely," he said.

CORRECTION: A previous headline contained an incorrect title for the Customs and Border Protection commissioner