Sensible way to rewrite a field for all views in Drupal 8?
You can use hooks the same way as in Drupal 7, if you want to change the content of the node before rendering, you can put a hook in your module or theme:

module/theme_preprocess(&amp;$variables, $hook) {
   if ($hook == 'node') {
      ...


check if you are in the right node and change it.

The simplest solution for your problem would be to put all children images in the node display and then hide the images you don't want:

hide($variables['content']['field_image_?']);


So you don't need to lookup the children image yourself.