Child themes only let you override templates, but they don’t override other PHP files, e.g. anything included from functions.php.
You can try to change things via hooks, but if the function provides no hooks then you’re stuck. There is no way to override it unless a way has been provided by the author of the parent theme.
Your options are:
- fork the parent theme
- contact the author
- Create your own function with a different name, then override every template that uses the original ( this one is a longshot, and won’t work if the function is used in a PHP file that isn’t a template )