It was "wrong" not to take further action against nurses involved in the care of a Londonderry girl who died of hyponatraemia, the Nursing and Midwifery Council (NMC) has said.
The death of Raychel Ferguson in 2001 was found to be "avoidable".
In October, the NMC said no further action was required after it reviewed the cases of five nurses who were named in the inquiry's report,
Failings had been addressed, it added.
However, it has now said it is sorry for that decision.
The reversal came after Raychel's family appealed against the decision and provided the regulator with additional evidence.
The NMC has said it will now send the referral for a full fitness to practise investigation.
Raychel's mother Marie told BBC News NI that the family "welcomed the news that the NMC have finally come to their senses and are investigating these nurses".
"The NMC has fully apologised and acknowledged the hurt that they have caused to our family by previously coming to the wrong decision."
The NMC emailed the family about its decision this week. It included their reasoning, which it said varies a little between each nurse.
In a summary, seen by the BBC, it said that "in addition to the clinical concerns previously identified, we now consider that Sir John's report raises attitudinal concerns, in relation to four of the five nurses".
It added that an investigation was needed to decided whether the nurses "failed to be open and candid about what went wrong in Raychel's care" and that "consequently those nurses' integrity may be in question".
Raychel was nine when she died at the Royal Belfast Hospital for Sick Children in June 2001, a day after an appendix operation at Altnagelvin Hospital in Londonderry.
An inquiry into the deaths of five children concluded in 2018 that four were avoidable.
A number of nurses and doctors named in the report referred themselves to the regulators.
Documents provided by the Western Health Trust to the NMC said concerns raised by the inquiry, including record training and training in preventing, recognising and managing hyponatraemia in children by the nurses, had all been rectified.
However, on reflection, the NMC said it should not have relied on this information from the trust and instead they required sufficient, independent documentary evidence - for example certificates from training providers - establishing that training had been undertaken and the basic nursing failures remedied.
Raychel's mother Marie told BBC News NI that, because of the U-turn she now placed complete faith in them dealing with the matter in the way she felt it should have been dealt with in the first place.
In a statement to the BBC, Matthew McClelland, the director of fitness to practise at the NMC, said: "We have been in contact with Raychel Ferguson's family to let them know we have decided to investigate the conduct of five nurses involved in the care of their daughter.
"I am sorry we did not make the right decision the first time and am in no doubt of how upsetting that was for Raychel's family.
"I hope our decision today will go some way in reassuring them that their concerns about Raychel's care will be fully investigated."