Belfast Health Trust has apologised and paid out £6,000 for "significant failures" in its care for a dying patient.
The girl's father was left to carry out basic nursing tasks, including administering fluids, with no assistance. He also said he was not consulted by staff on the ward.
Public Services Ombudsman Marie Anderson said closer observation may have identified her deterioration sooner and allowed the girl's mother to arrive in time to support her daughter in her final moments.
The watchdog's report said: "The Ombudsman found significant service failures in the care provided, most notably in relation to an unreasonable responsibility placed on the father to attend to his daughter during the night.
"She concluded if the nursing staff had discussed with the parents their expectations of their daughter's care and treatment these failings may not have occurred."
Although the investigation did not determine whether the failings identified contributed to the patient's death, an uncertainty remains whether closer and more regular observation would have identified a deterioration in her condition at an earlier stage.
"This may have allowed for the girl's mother to be alerted in order for her to arrive at the hospital in time to support her daughter in her final moments."
The health trust had carried out its own internal investigation into what happened and subsequently commissioned an "independent" review panel to examine the complaint.
The Ombudsman found that its independence was "miscommunicated" to the girl's relatives and the report was amended by senior trust staff prior to its communication to the complainants to omit earlier criticisms.
In an unrelated case, the Ombudsman received a complaint from a family about the actions of the Western Health & Social Care Trust after a post-mortem examination revealed that their mother had sustained multiple fractures shortly after her death.
Although the trust had classed it as a serious adverse incident and initiated a critical incident review there were insufficient interviews conducted with staff who may have been involved and clear evidence of a "closed mind" attitude throughout.
Discussions with other trust staff were conducted by ward staff whose own actions were the subject of its internal investigation.
Staff did not record any details of the after-death care provided to the deceased.
The Ombudsman was satisfied that the fractures were not caused deliberately or as a result of any inappropriate intervention by any person involved in the handling of the deceased's body.
Her office said: "The investigation report concluded that the multiple failures by the trust fundamentally undermined the integrity, effectiveness and independence of the critical incident review."
