(Formerly User:Ciceronianclausula)
The main article hatnote always determines the relationship between a section and an article. What it says is: "there is an article matching exactly the subject of the section you are reading". So Child is not the "main article" of any section in Child mortality, unless there was a pointlessly off-topic section about what children are.
The best piece of guidance can be found on the template's documentation page, here Template:Main. As it says, the template is quite often misused. Often Template:Further information is intended. Often it's worth changing the "main article" template to this "further information" one.
The main article hatnote determines the relationship between a section and an article, so it's relational, and not absolute. The "main article" of a section called "Causes" in World War II is Causes of World War II, although it's evident that World War II is the larger topic. Hierarchical, absolute organization is represented by Wikipedia:Categories. It's confusing because the "main" article is often something that is hierarchically below the article you are reading. (Finnusertop)