When immigrant children first cross the border into the United States, the first thing federal authorities do is determine if they have family inside the country they can live with until they're processed through immigration court system.
That will still be an option for some children and their relatives, but Trump's Department of Homeland Security (DHS) wants to limit their options.
More specifically, DHS wants to begin performing immigration checks on anyone else that may be living with their relatives and collect invasive biometric data from them.
From The Daily Beast:
[The] Department of Homeland Security proposed a rule that would have Immigration and Customs Enforcement (ICE) check the immigration status of sponsors looking to care for these children, as well as the immigration status of any other adults who live in the sponsors’ homes. The rule would have ICE get biometric data—potentially meaning fingerprints or retinal scans—of those individuals.
Because the relatives of undocumented children are sometimes themselves undocumented, immigrant rights advocates warn that the new rule could put some potential sponsors in fear of deportation, discouraging them from coming forward to take in unaccompanied children.
Even if you agree with the Trump regime's immigration policy, which I clearly don't, you should know that the alternative to allowing these children to live with their relatives is placing them in foster care or in shelters run by the Department of Health and Human Services (HHS).
Family is free. Foster care and HHS shelters are not free.
In addition to being needlessly cruel and invasive, this policy is also wasteful as it directly increases costs for the federal government to provide food, shelter, education, and health care to kids kept in federal custody as required by law.
You may consider all of this to be ironic because there's nothing Republicans hate more than "welfare" and funding children's health insurance programs. In this case, Trump will intentionally increase the cost of providing for children who could otherwise live with family.
I suppose the difference in this case is they can funnel this money directly to private entities contracted by the government to operate shelters.