Not a huge issue--I could live as it is--but this if/else dealio could all be reduced to `$batch = islandora_basic_collection_migrate_children_batch(
  $collection, $new_collection, (($clicked_button == 'submit_all') ? NULL : array_keys(array_filter($form_state['values']['children']))));`

Might still want to break it up into separate variables, instead of the having the ternary statement inline.