If the parent-Theme function in question isn’t either pluggable (i.e. wrapped in a if( ! function_exists( $function ) ) conditional) or filterable (i.e. returns its output wrapped in a apply_filters( $filtername, $output )), then there’s no easy way to override the function.
You’ll have to do one of the following:
- Replace all template files that contain instances of the function call, and replace those calls with your own child-Theme function, in all such templates
- Modify the parent Theme, to delete (or make pluggable) the function in question