"Grown children" is interesting if you look at google books cites - it more typically meant children older than infants than someone's children that have reache

"Grown children" is interesting if you look at google books cites - it more typically meant children older than infants than someone's children that have reached adulthood. "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.