There has to be a divorce or separation agreement first. He can't just take them and go. For a start, a single parent with children leaving a country may well be investigated by emigration/immigration authorities to see if the children are wards of court, and the travelling parent may be asked to show a letter of permission from the other parent to take the children abroad. The father may be turned back at the airport, or even arrested if he seems to be kidnapping the children. If Canada has Ward of Court proceedings, like the UK, it may end up that the children are made wards of court and a court hearing has to be set. Does Canada have anything like our Citizens Advice Bureaux ? He may have to find some semi-official advice organisation of that kind.