You should be able to find all macros defined in your wiki at https://<your.wiki.server.name>/xwiki/bin/view/XWiki/XWikiSyntaxMacrosList
There are macros that are defined in wiki pages; their names are linked to that page in the leftmost column. Others are not linked, as they are defined in Java components. The latter should be available in all wikis, unless they come from an extension only installed in the main wiki.
The “children” macro is defined in an XWiki page. (I think, the “example” macro, too.) Click on the link pointing to the page which defines that macro. Try to edit that page. You should get a warning that this macro belongs to some extension. (If you get no warning, press “Cancel” instead of editing the page.)
The name of the extension shown should tell you what to install in your subwikis to get the same macro in the subwiki, too.