I have two CPTs; Campaigns and Supporters. Supporters are parents of Campaigns in a one to many relationship.
I set up Campaigns to not have the option to have a featured image (in the toolset>post types options)
When I create a view in a block for campaigns but choosing the parent supporter as the source, the only image I can use in the block is the Supporter's author image; there is no option for the Supporter's featured image in the dropdown for the dynamic source.
Supporters DO have featured images enabled so this isn't expected.
The workaround is to enable featured images for Campaigns. This results in Supporter featured images being shown in the dynamic source dropdown as an option.
I did a quick check on this for you and you are correct in that you are not able to display the Parent's featured image on the child using the dynamic sources.
However you should be able to do it by using the shortcode [wpv-post-featured-image] and using the item attribute.