Jim, the only allegation is that they were buried in an unmarked communal grave.
There is no suggestion of unregistered deaths. No exhumations are required.
Your claims were baseless.
Here is the story again. Read it this time!
https://www.theguardian.com/uk-news/2017/sep/10/smyllum-park-lanark-orphanage-catholic-nuns-children-mass-grave-allegedly

"An inquiry by BBC Radio 4's File on 4 and the Sunday Post newspaper examined death certificates in archives and found 402 children from Smyllum Park."

"Records reveal that most of the deaths were due to natural causes, mainly from diseases such as TB, pneumonia and pleurisy. About a third of the victims were under the age of five, and the majority of the deaths occurred between 1870 and 1930."