The WP_DEFAULT_THEME constant actually just overrides what WordPress sets as default theme (which per default is the latest of the bundled themes). It does not really help you to define your parent theme. In fact you just have to “Network enable” your parent and child themes and then set the parent theme in the child themes header. No need to do more than that:
/**
* Theme Name: Awesome Child Theme Name
* Template: parent-theme-folder-name
* ...
*/