There's a McFadyen family in Edinburgh in 1871
Alexander McFadyen, coachman, 36, born Edinburgh
Christian McFadyen, wife, 39, born Glasgow
William McFadyen, son 9, born Edinburgh
John Brady, son, 19, born Bathgate
1n 1881, still in Edinburgh
Alexander McFadyen, 44, annuitant, born Edinburgh
Christina McFadyen, 50, born Glasgow
William McFadyen, 19, born Edinburgh
This couple is Alexander McFadyen and Christina Munro.
Christina Munro, wife of Alexander McFadzean, died in Edinburgh in 1881.
What this means is that (surprise, surprise - not) the Ancestry trees are wrong. The couple in the 1861 census in Edinburgh are not your Alexander and Christian, and William, born 24 August 1861 in Edinburgh, has nothing to do with your family, who were in Australia by 1871.