www.scq.ubc.ca/the-truth-about-chimeras/
"Most human chimeras, however, are not even aware of their conditions, because many of them appear completely normal. The most famous cases of chimerism to date are the linked cases of Lydia Fairchild and Karen Keegan. Fairchild was pregnant with her third child when she separated with her partner, James Townsend. In order to obtain state welfare, she had to prove that she was the biological mother of her two born children. It was discovered, through DNA testing, that it was impossible that she was the biological mother of her two children because she bore no genetic similarity to them whatsoever. A case of welfare fraud ensued because the prosecutors believed the DNA results to be irrefutable. Even the testimony of Dr. Leonard Dreisbach, the obstetrician who had helped Fairchild give birth, did little to persuade the court in Fairchild?s favor. The judge, perplexed by seemingly conflicting evidence, ordered that the third child, when born, to be tested as well. Surprisingly, the third child also showed no genetic similarities as well.
Fortunately for Fairchild, Karen Keegan also had similar experiences. Keegan needed a kidney transplant, and DNA testing for a compatible match with her two eldest sons showed that she had no genetic similarities to them at all. However, the doctors who worked with Keegan were familiar with the concept of chimerism and suggested that Keegan undergo further testing. Testing of her brothers and husband proved that her sons were related to them. Subsequent sampling of her skin and hair proved to be futile, but eventually matching DNA was found in her thyroid gland. It was the publication of this case, in the New England Journal of Medicine, which offered new insight on the case of Lydia Fairchild. Fairchild was found later on to be a chimera, with the second set of DNA found from her cervical smear. It was concluded that both Keegan and Fairchild were tetragametic chimeras.
These cases challenge the blind faith which the scientific community places on the irrefutability of DNA testing. "
So as well as the reliability of the actual test - percentage of false positives/negatives and the reliability of the lab - cross contamination chain of evidence ect also have to explain the rare cases where DNA may not match to people undergoing tests - as the consequences of getting things wrong is so huge.
I would have though the data obtained by testing and how to interpret the information ( think it expressed as statistical probability) and its reliability and any implications to parents and DC would best be handled via counselling. Still I have never had the kind of lifestyle that would require this kind of testing ( despite what my MIL has occasionally tried to suggest despite all evidence to contrary)