I really dislike the term "adult children". Check out the google ngram:
https://books.google.com/ngrams/graph?content=%22adult+child...
The peak in 1992 is especially intriguing.
How would the Victorians have expressed the same relationship?
The peak in 1992 is especially intriguing.
How would the Victorians have expressed the same relationship?
"Offspring", maybe, but you can look at the differences in use and decide for yourself. It might confound you to find that "adult offspring" is less common than "adult children" but follows the same shifts in use and a similar ngram pattern.
Second: If you must, "son" or "daughter", and only prefixed by "adult" when necessary for clarity.
To your second point: "son" or "daughter" is not sufficiently general, and it is gendered. What if I wanted to express the idea of adult offspring without knowing (or caring about) the gender of that person?