On Tuesday, a BBC Spotlight programme revealed that eight children had gone missing in Northern Ireland.
Two of the girls were Somali teenagers who disappeared in 2005 and 2012.
The Health and Social Care Board has said new guidelines are in place and add that no children have gone missing since 2014.
Separated children are children outside their country of origin and separated from their parents or legal guardian.
The term can also include unaccompanied asylum-seeking children and trafficked children.
When they arrive in Northern Ireland they are taken into the care of the local health trust.
Eight children have gone missing since 2005 and they remain missing.
The SDLP's health spokesman Mark H Durkan said he would be raising the issue at the Northern Ireland Assembly's health committee and his party colleague Alex Attwood would be raising it at the justice committee.
"The number of children who cannot be accounted for is something that needs urgent inquiry and investigation," he said.
"There is a lot of very good work being done to look after the welfare of unaccompanied young people, but clearly we now have some very big questions that need to be answered."
Ulster Unionist MLA Jo-Anne Dobson said it was "frankly appalling" to hear that eight children had gone missing.
"I have written to Health Minister Michelle O'Neill on this issue to seek further clarification and to demand details of how the department, health trusts and the Health and Social Care Board have sought to address each of the cases involved in the investigation," she added.
The Green Party leader Steven Agnew also said it was extremely worrying that children can disappear without a trace.
Paula Bradshaw from the Alliance Party added that the health trusts and police "need to work closer over the handling of these cases".
In a statement, the Police Ombudsman for Northern Ireland said: "Our director of investigations will be reviewing the contents of the programme to ascertain if there are any issues of police conduct which may need further investigation."
The Police Service of Northern Ireland has said that in the two cases identified in the programme, investigations were robust and all information available at the time was followed.
The Health and Social Care Board has said that new guidelines are in place and stress that no children have gone missing since 2014.
BBC Spotlight's investigation is now available on BBC iPlayer.
An "urgent inquiry" is needed into separated children who have gone missing from care, the Social Democratic and Labour Party has said.