Some more together.

src/Form/AddChildrenWizard/FileSelectionForm.php

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+<?php
+
+namespace Drupal\islandora\Form\AddChildrenWizard;
+
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+class FileSelectionForm extends FormBase {
+
+  public function getFormId() {
+    return 'islandora_add_children_wizard_file_selection';
+  }
+
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    // TODO: Using the media type selected in the previous step, grab the
+    // media bundle's "source" field, and create a multi-file upload widget
+    // for it, with the same kind of constraints.
+    return $form;
+  }
+
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // TODO: Implement submitForm() method.
+    $form_state->setError($form, 'Oh no!');
+  }
+}

src/Form/AddChildrenWizard/Form.php

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+<?php
+
+namespace Drupal\islandora\Form\AddChildrenWizard;
+
+use Drupal\ctools\Wizard\FormWizardBase;
+
+class Form extends FormWizardBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getOperations($cached_values) {
+    // TODO: Implement getOperations() method.
+    return [
+      'child_type' => [
+        'title' => $this->t('Type of children'),
+        'form' => TypeSelectionForm::class,
+      ],
+      'child_files' => [
+        'title' => $this->t('Files for children'),
+        'form' => FileSelectionForm::class,
+      ]
+    ];
+  }
+}

src/Form/AddChildrenWizard/TypeSelectionForm.php

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+<?php
+
+namespace Drupal\islandora\Form\AddChildrenWizard;
+
+use Drupal\Core\Cache\CacheableMetadata;
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityTypeBundleInfoInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Form\FormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\islandora\IslandoraUtils;
+use Drupal\taxonomy\TermInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+use function Symfony\Component\DependencyInjection\Loader\Configurator\iterator;
+
+class TypeSelectionForm extends FormBase {
+
+  protected ?CacheableMetadata $cacheableMetadata = NULL;
+  protected ?EntityTypeBundleInfoInterface $entityTypeBundleInfo;
+  protected ?EntityTypeManagerInterface $entityTypeManager;
+  protected ?EntityFieldManagerInterface $entityFieldManager;
+
+
+  public static function create(ContainerInterface $container) {
+    $instance = parent::create($container);
+
+    $instance->entityTypeBundleInfo = $container->get('entity_type.bundle.info');
+    $instance->entityTypeManager = $container->get('entity_type.manager');
+    $instance->entityFieldManager = $container->get('entity_field.manager');
+
+    return $instance;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'islandora_add_children_type_selection';
+  }
+
+  protected ?array $nodeBundleOptions = NULL;
+  protected ?array $nodeBundleHasModelField = NULL;
+  //protected ?array $nodeBundleHasMemberOfField = NULL;
+  protected function getNodeBundleOptions() : array {
+    if ($this->nodeBundleOptions === NULL) {
+      $this->nodeBundleOptions = [];
+      $this->nodeBundleHasModelField = [];
+      //$this->nodeBundleHasMemberOfField = [];
+
+      $access_handler = $this->entityTypeManager->getAccessControlHandler('node');
+      foreach ($this->entityTypeBundleInfo->getBundleInfo('node') as $bundle => $info) {
+        $access = $access_handler->createAccess(
+          $bundle,
+          NULL,
+          [],
+          TRUE
+        );
+        $this->cacheableMetadata->addCacheableDependency($access);
+        if (!$access->isAllowed()) {
+          continue;
+        }
+        $this->nodeBundleOptions[$bundle] = $info['label'];
+        $fields = $this->entityFieldManager->getFieldDefinitions('node', $bundle);
+        //$this->nodeBundleHasMemberOfField[$bundle] = array_key_exists(IslandoraUtils::MEMBER_OF_FIELD, $fields);
+        $this->nodeBundleHasModelField[$bundle] = array_key_exists(IslandoraUtils::MODEL_FIELD, $fields);
+      }
+    }
+
+    return $this->nodeBundleOptions;
+  }
+
+  protected function getModelOptions() : \Traversable {
+    $terms = $this->entityTypeManager->getStorage('taxonomy_term')
+      ->loadTree('islandora_models', 0, NULL, TRUE);
+    foreach ($terms as $term) {
+      yield $term->id() => $term->getName();
+    }
+  }
+
+  protected function mapModelStates() : \Traversable {
+    $this->getNodeBundleOptions();
+    foreach (array_keys(array_filter($this->nodeBundleHasModelField)) as $bundle) {
+      yield ['value' => $bundle];
+    }
+  }
+
+  protected ?array $mediaBundleOptions = NULL;
+  protected ?array $mediaBundleUsageField = NULL;
+  protected function getMediaBundleOptions() : array {
+    if ($this->mediaBundleOptions === NULL) {
+      $this->mediaBundleOptions = [];
+      $this->mediaBundleUsageField = [];
+
+      $access_handler = $this->entityTypeManager->getAccessControlHandler('media');
+      foreach ($this->entityTypeBundleInfo->getBundleInfo('media') as $bundle => $info) {
+        $access = $access_handler->createAccess(
+          $bundle,
+          NULL,
+          [],
+          TRUE
+        );
+        $this->cacheableMetadata->addCacheableDependency($access);
+        if (!$access->isAllowed()) {
+          continue;
+        }
+        $this->mediaBundleOptions[$bundle] = $info['label'];
+        $fields = $this->entityFieldManager->getFieldDefinitions('media', $bundle);
+        $this->mediaBundleUsageField[$bundle] = array_key_exists(IslandoraUtils::MEDIA_USAGE_FIELD, $fields);
+      }
+    }
+
+    return $this->mediaBundleOptions;
+  }
+
+  protected function getMediaUseOptions() {
+    /** @var TermInterface[] $terms */
+    $terms =  $this->entityTypeManager->getStorage('taxonomy_term')
+      ->loadTree('islandora_media_use', 0, NULL, TRUE);
+
+    foreach ($terms as $term) {
+      yield $term->id() => $term->getName();
+    }
+  }
+  protected function mapUseStates() {
+    $this->getMediaBundleOptions();
+    foreach (array_keys(array_filter($this->mediaBundleUsageField)) as $bundle) {
+      yield ['value' => $bundle];
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $this->cacheableMetadata = CacheableMetadata::createFromRenderArray($form);
+
+    $form['bundle'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Content Type'),
+      '#description' => $this->t('Each child created will have this content type.'),
+      '#options' => $this->getNodeBundleOptions(),
+      '#required' => TRUE,
+    ];
+
+    $model_states = iterator_to_array($this->mapModelStates());
+    $form['model'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Model'),
+      '#description' => $this->t('Each child will be tagged with this model.'),
+      '#options' => iterator_to_array($this->getModelOptions()),
+      '#states' => [
+        'visible' => [
+          ':input[name="bundle"]' => $model_states,
+        ],
+        'required' => [
+          ':input[name="bundle"]' => $model_states,
+        ],
+      ]
+    ];
+    $form['media_type'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Media Type'),
+      '#description' => $this->t('Each media created will have this type.'),
+      '#options' => $this->getMediaBundleOptions(),
+      '#required' => TRUE,
+    ];
+    $use_states = iterator_to_array($this->mapUseStates());
+    $form['use'] = [
+      '#type' => 'checkboxes',
+      '#title' => $this->t('Usage'),
+      '#description' => $this->t('Defined by <a target="_blank" href=":url">Portland Common Data Model: Use Extension</a>. "Original File" will trigger creation of derivatives.', [
+        ':url' => 'https://pcdm.org/2015/05/12/use',
+      ]),
+      '#options' => iterator_to_array($this->getMediaUseOptions()),
+      '#states' => [
+        'visible' => [
+          ':input[name="media_type"]' => $use_states,
+        ],
+        'required' => [
+          ':input[name="media_type"]' => $use_states,
+        ],
+      ],
+    ];
+
+    $this->cacheableMetadata->applyTo($form);
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    // TODO: Implement submitForm() method.
+  }
+}