They are believed to be the children of prostitutes living in Roman Britain who were victims of infanticide.
Said one researcher:
In Roman times, "we know newborn babies weren't viewed in the same light as older babies and children as they were growing up," Mays said.
In order to achieve a fully human status, babies needed to survive to about 6 months old, Mays said.
This is not the first statement I've read about the lack of humanity of the newborn.
In Joseph Dellapenna's book Dispelling the Myths of Abortion History, he writes that in the Early Modern era, it was commonly believed that a baby was not a human being until baptism.
Infanticide was very common in certain parts of Europe, and was a main method of limiting family size, and it was practiced by women in various circumstances, not just the destitute. Although the law may have recognized that it was a crime, popular belief rationalized it to such an extent that in many communities, ten, twenty, thirty or even forty percent of newborns were believed to have been allowed to die (or were deliberately killed) in order to limit family size. Although a baby could have been easily smothered and died without suspicion in an age of high infant mortality, especially if the father vouched for the mother, infants were often killed by exposure, e.g. abandoned on a city street or a church porch, or they were sent to wet nurses who were not paid and who then subsequently allowed the baby to die of neglect or "overlaying" i.e. rolling over the child.
Since there was a lack of forensic science in this age, and child mortality was high, it was very difficult to prove that infanticide had occurred, and given the common beliefs about the nature of humanity of the newborn, it was difficult to convict a woman of the crime, especially if it was a capital crime. Jurors tended to sympathize with the mother.
Pro-life discourse on this subject might be countered by raising the objection that the emergence of abortion in the 19th and especially the 20th century made infanticide virtually obsolete. However, if infanticide was acceptable to many based on the dehumanization of the unborn, abortion was also permitted to spread in the same context (and in age which was extremely ignorant of fetal development).
I think the link between the dehumanization of the infant and the dehumanization of the unborn and how they lead to killing might be an easier analogy to suggest than the analogy between the dehumanization of the unborn and that of Blacks, Jews, Native Americans and Women. For one thing, infants were killed for the very same reason that the unborn are killed today.
The downside of this analogy is that, as far as I know, no western government in the modern era has ever denied the humanity of the infant. This was done in pagan societies such as pre-Christian Rome, where the Father had the right to expose his infant child. Nonetheless, the parallels between infanticide and abortion are rather striking. I think pro-lifers might want to explore this topic further.