Collection and Use of Biometrics by U.S. Citizenship and Immigration Services
federalregister.gov
federalregister.gov
In order to keep it legal and above board, the US state assumes they are not her kids until proven otherwise. So yes, a DNA test is required. The other stipulation is a state department person needs to be in the room with the kids blood is drawn. A lot of red tape!
On the one hand the whole thing seems aggressive and strict. On the other - you can imagine that these rules went in place from bad experiences i.e. the DNA test existed because people were scamming the "bring children over", and then a state department person needed to be present because people were scamming the blood draw. It's another example of a few bad apples ruining it for everyone.