Do we have any idiom for creating one or more item resources at once?, without affecting existing sibling resources?

Exactly the same

The 201 (Created) status code indicates that the request has been fulfilled and has resulted in one or more new resources being created. The primary resource created by the request is identified by either a Location header field in the response or, if no Location header field is received, by the target URI. -- RFC 9110

Emphasis added

So single POST Request, server produces multiple resources, HTTP response headers identify the primary resource created, is all straight forward.

The response body... I don't think there are any fixed standards on how to describe the created resources in the response body. On the web, it would normally just be an HTML page showing the human a bunch of links....