multi.MultiValueSubChildrenNodeTransformer can now be configured in Magnolia 6 UI as follows:

Use info.magnolia.ui.field.JcrMultiFieldDefinition and set:

• JcrMultiFieldDefinition#field to $type: compositeField
• CompositeFieldDefinition#properties to a single field that shares the same name with the JCR multi field

relatedUUID:
  $type: jcrMultiField
  field:
    $type: compositeField
    properties:
      - name: relatedUUID
        $type: linkField
        datasource: *categoryDatasource

Page to update:

https://docs.magnolia-cms.com/product-docs/Templating/Dialog-definition/Field-definition/Magnolia-6-UI-ports-of-5-UI-field-transformer-classes.html