We should show the "margins" (left/right and top/bottom) applied to the child inside the Canvas.
Also, it would be nice to have a similar behavior as with the Grid when dragging a child, so it updates Canvas.Left or Canvas.Right properties depending if the element is near the left or right edges.
And in that case, we should have a "lock" toggle to allow the user to anchor the child to one of the sides, so dragging the child will always update the locked property.