What is the row over ‘missing’ kids in US?

In April, The New York Times reported data from the US Department of Health and Human Services (HHS) that showed that since October 2017, more than 700 migrant children had been taken from adults claiming to be their parents, including more than 100 children below age 4. The administration of President Donald Trump, for whom illegal migration has been a core agenda issue, had earlier said it might take children from parents to deter illegal border crossings, and lawmakers had been asking for numbers of families being separated along the US’s southern border. It was subsequently reported that nearly 1,500 immigrant children had been “lost” while in federal custody until the end of last year.Over the past several days, a heated public debate has raged over the administration’s policy towards migrant children. There has been outrage and calls to action in civil society and the liberal commentariat, and social media has been aflame with hashtags such as #WhereAreTheChildren and #MissingChildren. What are the issues here, and why have they exploded now?
On April 26, an HHS official told a committee of the Senate that his department did not know the whereabouts of 1,475 children who had entered unaccompanied from Mexico, and had been placed with adult sponsors in the US. Between October and December 2017, HHS had found that out of 7,635 children it had handed over to sponsors, 6,075 were still there, 28 had fled, 5 had been deported, and 52 were living elsewhere, the AP reported. The other1,475 were missing.
HHS has said it has no legal responsibility to trace these children. It has also been argued that some sponsors, possibly relatives, may be blocking attempts to reach the children. Critics.