when you start doing a LOT of customization in your child theme, you can end up with a monster-sized functions.php that is difficult to navigate. So instead of going crazy looking for functions, I split them up into their own files, put them in a special folder, and then just require them in unctions.php. best practice? i have no idea, but it works for me and helps me stay organized. here is what i do:
1. create a folder in my child theme called "functions"
2. in functions.php create a constant that defines the location of this folder
define('FUNCTIONS_PATH', STYLESHEETPATH . '/functions/');
do you need a constant? well not really, but if you start adding a lot of extra function files it is nice... and if you happened to move/rename stuff you only have to change the value of the constant and not try to remember everywhere you've used it.
3. put your custom functions (in this exmaple i will create a file for menu functions where i typically declare all the menu locations, arguments, etc) in their OWN php file and then store it in your functions folder.
4. in your functions.php add the following (obviously depending on what you called the file in step 3
require_once(FUNCTIONS_PATH . 'menu-functions.php'); // Custom menu functions
you can use this multiple times and just require multiple files. I usually use for menu-functions, my exceprt functions/class, theme-functions (post header/footer type stuff), and all my admin/backend stuff. this helps me sort the functions into groups which keeps me organized and not scanning through the thousands of lines of code that can end up in functions.php if you start to get pretty involved.
hope this helps... enjoy.