Generative Fill increasingly blocked on images of children due to stricter guidelines

What do you mean by »fine-art portrait«? A simulation of some painterly style or of a certain type of photography? 

As ​@D Fosse elaborated images of children may be subject to specific legal attention/caution. 

If »Select Subject« provides a decent enough mask you might be able to change backgrounds with »traditional« methods or possibly by employing work-arounds like blacking-out elements or splitting the images – which is obviously not terribly efficient. 

What kind of backgrounds do you usually switch into the photographs – fairly plain gradients/soft studio backgrounds or more elaborate scenarios. 

For obvious reasons, legal protection concerning photographic reproduction of children has become very much stricter in recent years. In the EU area we have this legal framework called GDPR (General Data Protection Regulation) that regulates this. I don’t think that applies in the US, but there is probably something equivalent/similar.