Parent term suggestion on entering the child term in term reference field?
I think you'll need to override the taxonomy autocomplete path. Here's what you will need:


hook_menu_alter() implementation, so your custom module takes over the taxonomy autocomplete suggestions. See hook_menu() implementation, specifically $item['taxonomy/autocomplete'] item in /modules/taxonomy/taxonomy.module for more information.
You'll need to re-create the core taxonomy_autocomplete() function, which is in /modules/taxonomy/taxonomy.pages.inc.
You'll have to perform a check and see for which vocabulary you need to show suggestions:


If it's the vocabulary where you want to have child -> parent functionality, add the code for fetching the parent term and provide additional suggestions.
If it's some other vocabulary, make sure you return the same output as the core function would.



You'll need to add this in a custom module.

function YOUR_MODULE_menu_alter(&amp;$items) {
  // This will re-route this path to your custom module, so you can have any
  // additional logic.
  $items['taxonomy/autocomplete']['page callback'] = 'YOUR_MODULE_taxonomy_autocomplete';
}

function YOUR_MODULE_taxonomy_autocomplete($field_name = '', $tags_typed = '') {
  // Copy the content of taxonomy_autocomplete() function here, and add necessary
  // modifications at the end of it.
}


Hope this helps.