The Nursing and Midwifery Council (NMC) told Marie Ferguson the failings identified have been addressed.
It said no further action was required at this time.
The Hyponatraemia Inquiry found four out of the five deaths it investigated were preventable.
Hyponatraemia is an abnormally low level of sodium in blood and can occur when fluids are given incorrectly.
The damning report was heavily critical of the “self-regulating and unmonitored” health service.
Raychel Ferguson was nine-years-old when she died at the Royal Belfast Hospital for sick Children (RBHSC) in June 2001, a day after an appendix operation at Altnagelvin Hospital, Londonderry.
Four other children – Adam Strain, Claire Roberts, Lucy Crawford and Conor Mitchell – all died at the RBHSC between 1995 and 2003.
In a letter to Raychel’s mother Marie Ferguson the NMC – the regulatory body for nurses – said that while the allegations were “extremely serious” they had “concluded that the failings identified have been addressed so there is no further action they could take at this time”.
Mrs Ferguson said she was “extremely disappointed” and called on the NMC to reverse its decision.
In a statement, an NMC spokesperson said: “It’s clear that there were serious failings in care around hyponatraemia at Western Health and Social Care Trust and that these failings have had a devastating impact on the families involved.
“All of the nurses have undertaken training around recognising and managing hyponatraemia and the trust have made changes on the wards to assist with recognising and escalating concerns.
“We understand the impact of our decision on the family. We have been in contact with them and look to remain in contact with them throughout this difficult time.”
The 14-year inquiry was chaired by Mr Justice O’Hara and his final report was scathing, both of how the families had been treated in the aftermath of the deaths and of medical professionals’ attitude to his investigation.
He said some medical witnesses to the inquiry “had to have the truth dragged out of them”.
Mr O’Hara’s report made 96 recommendations – the first of which was that a statutory duty of candour be set up, breach of which would be a crime.
In a letter to Mrs Ferguson dated October 15th, the NMC said it took it seriously when nurses’ actions have led to or contributed to death or serious injury.
But the NMC said its focus was on whether the nurse is likely to put patients at risk in the future, adding that the process is “not intended to punish nurses” and that they want to “encourage an open culture where patient safety concerned are reported.”
The letter notes the evidence given by nurses to the public inquiry and that there was “no suggestion that the nurses were aware of the risks of hyponatraemia or had appreciated risks associated with Raychel’s condition.”
Therefore it was the NMC’s view that “the nurses did not appear to knowingly take a risk with Raychel’s condition.”