if she's exploring and critiquing attitudes about adoption that were more common in her day and we've forgotten.

I think a bit of both. The modern Western concept of adoption is very recent and local (which is one of the reasons international adoption can get so terrible, with living parents who gave their children up thinking it'd be something more like sending them to school or an apprenticeship, not permanently giving up all parental connection to their child and having someone else take on that role entirely.)

But I wouldn't ever want to bet against Christie being prejudiced herself, or uncritically reflecting the prejudiced attitudes of her society.