You can tell everything there is to know about the parents and children just by looking in the document tree (MODx makes use of the "parent" field when it’s drawn). You can also change the parent by dragging and dropping documents or by editing them and clicking on the Resource Parent icon.
If you have Friendly URL’s set up properly (or turned off), you don’t ever need to type a URL, just use a link tag with the id of the document you want to link to. MODx should automatically create the appropriate URL and you can see what it is.
Here’s a link to document 12:
<a href="[~12~]">Go to Doc 12</a>
BTW, have you tried the Wayfinder snippet? I will create menus with links to documents automatically.
If you put the following in your template, you’ll see a menu with links to all your pages. You can check the URLs MODx is using by hovering over them.
[[Wayfinder? &StartId=`0`]]