ICE Turning U.S. Citizens Over to DHHS? Are Those under Five of "Unknown Parentage" U.S. Citizens?

A reporter from Buzzfeed, Amber Jamieson, brought to my attention a passage from today's Declaration indicating that at least one U.S. citizen may be in the custody of Department of Health and Human Services.

Her article states:

A child under the age of 5 remains detained by the federal government after being separated from their parent at the US–Mexico border more than a year ago, even though they may be US citizens [sic].

8 CFR 1240.8 says that burden on the government to prove by "clear and convincing evidence" that someone put into removal proceedings is indeed an alien.

Today's declaration states:

One child on the original list has a parent who may or may not be a United States citizen (insufficient information is available to make this determination, and the parent and others are not available to provide that information). The child was separated from her parent in 2015 when her parent was arrested on an outstanding warrant by the U.S. Marshals Service. Defendants have not been aware of the parent’s location since then and they remain unable to locate that parent. 

A child in a class of those under 5 years old and taken into custody in 2015 could have been no more than 2 years old at the time of the separation.  If that child has been incommunicado from any relatives, then the child would have no information on where she was born.  Assuming that the child had no identification at the time her parent was arrested on an outstanding warrant -- which implies that the parent had been living in the U.S. for enough time to accumulate an outstanding warrant -- then it seems not only unlikely that ICE would have evidence of the child's alienage but also likely that the child was born in the United States.

What happened to this parent?  The U.S. Marshals could find the parent because of an outstanding warrant but a judge's order to do so leaves them coming up short? 

"Foundling" Law
Another possible scenario is that the government does not know who the child's parent even is.  This has been broached in some cases but I have not heard anyone make the point that each and every one of such individuals is by law a U.S. citizen.

According to 8 USC 1401, "The following shall be nationals and citizens of the United States at birth":

a person of unknown parentage found in the United States while under the age of five years, until shown, prior to his attaining the age of twenty-one years, not to have been born in the United States.

In short, the U.S. government can collect all the DNA that it wants, but if they cannot match the children with a known parent, and cannot prove they were not born in the United States, then these children are effectively of "unknown parentage found in the United States" and they all are legally  U.S. citizens at birth.

UPDATE 9:15 pm: Amber Jamieson posted on Twitter stating that ICE says the possible US citizen parent in question presented herself and her son "born in Mexico" at the border in 2015 and that she was then taken into custody because of an outstanding warrant.  Once more a credible journalist  repeats ICE's claims without a shred of evidence or verification.  If ICE has hard evidence of this, why wasn't it in the Declaration, one necessary to show compliance with a court order? Why not release the documents?  The same government that claims it cannot now locate the mother expects, alas correctly, a compliant media will reprint their assertions about her background, just because they said so.
Why are reporters continuing to print statements from official ICE-dom when that agency has been demonstrably lying about US citizens detained and deported for decades?  Maybe this time ICE is right but the media is supposed to print the truth that it has verified, and not amplify whatever propaganda the government feeds them.