The union representing Manitoba's child-welfare and social workers is questioning whether the abuse and murder of a five-year-old girl should be put to a public inquiry.

Phoenix Sinclair, seen in an undated photo, was discovered dead in March 2006 on the Fisher River First Nation. A public inquiry into her death is slated to begin May 23. (Family photo)

Lawyers for the Manitoba Government and General Employees' Union (MGEU) appeared before the province's Court of Appeal on Thursday, as part of the union's bid to block the inquiry into the 2005 death of Phoenix Victoria Hope Sinclair.

Ordered by the province in 2006, the Sinclair inquiry is set to begin on May 23. It aims to examine how Manitoba's Child and Family Services staff and officials failed to protect the five-year-old girl, who was killed by her mother and stepfather in June 2005.

The inquiry will also examine how Phoenix's death went unnoticed for months. It wasn't until March 2006 that her body was found wrapped in plastic in an unmarked shallow grave on the Fisher River First Nation.

But John Harvie, a lawyer for the MGEU, argued on Thursday that deaths of children in foster care are normally examined by provincial court inquests, not public inquiries.

Phoenix had spent much of her young life in foster care before her mother, Samantha Kematch, regained custody of her in 2004.

The little girl died after neglect and repeated abuse by Kematch and her common-law husband, Karl McKay, both of whom were convicted of first-degree murder in 2008.

Court heard that since 2003, there have been six deaths of children in the Child and Family Services system. An inquiry was not called in any of those cases, according to the union.

Harvie has asked the court to require Ted Hughes, the Sinclair inquiry's commissioner, to prove that he has the authority to hold the inquiry.

Manitoba government lawyer Heather Leonoff argued that the province has the right to call an inquiry, which is much wider-ranging than an inquest and can look at systemic issues in child care.

Other methods of inquiry, such as an inquest or an investigation by a medical examiner or children's advocate, would not have the same powers as a public inquiry, including the power to subpoena witnesses, according to the province.

The government asked the court not to delay the Sinclair inquiry any longer, arguing that it's in the public interest.

Justice Martin Freedman has reserved his decision, and indicated it will be delivered before the end of next week.

With files from the CBC's Marisa Dragani and The Canadian Press