Tens of thousands of Christians were forced to flee their homes in the area in June 2014 when IS militants took control.
The extremists ordered them to pay a tax, leave or face death just because they were not Muslim.
United Nations officials have said the organisation has evidence of "several cases of mass executions of boys, as well as reports of beheadings, crucifixions of children and burying children alive".
IS is systematically killing, torturing and raping children and families of minority groups, the UN said in a report on the Rights of the Child.
Iraq's second city is seen as a major stronghold of IS but Premier has learned the Iraqi army is preparing an attempt to take Mosul back.
Retired Major General Tim Cross said the UK was helping the country's force get ready to strike Mosul as soon as the Spring.
If it's taken back from IS it would be seen as a major achievement in the fight against the group.
Major General Cross said: "They're looking to try and put an offensive campaign together in the Spring or Summer.
"But are they ready to do that? Do they have the capability, the physical stuff, the understanding of how to fight the campaign?
"Are they coherent between their own forces?
"I think the government's view is we should support them and we have put people in to train them."
He added: "We need to make sure that when we go on the offensive, it's got to succeed."
Militants in the city have already faced air strikes from the US led coalition targeting the group.
Meanwhile a report by MPs has found the UK is contributing less than its partners in the fight against Islamic State.
The Commons Defence Select Committee says the British military also lacks a strategy for defeating IS, and Iraq should be a higher priority.
But Defence Secretary Michael Fallon rejected the allegations.
He told the committee yesterday: "There has been progress. There is no doubt that the advance of Isil has been halted and in some areas - just to the west and north of Baghdad and just along the front line of the Kurdish forces - there is some evidence now that Isil [IS] is beginning to suffer reverses, and obviously that is something we can measure.
"I can measure it not least in seeing where the strikes are that I am being asked to authorise. Where those start to fall you can see significant progress westward, which is important."