. . . by their grown-up children for breach of privacy.
Children have absolutely no say or control over what is shared by their parents, or with whom it is shared.
My family member has posted, on her public Instagram feed, her son's full name, date of birth, and every waking moment. Now that information will be out there for anyone to consume. At best, this could erode the boy's sense of self-esteem, security, and privacy. At worst, the data can be used to commit identity fraud or give a stranger the information they need to socially engineer access to him.
And I don't care if you say that you get consent before you post anything, because children don't have full knowledge of the context of their decision. They are not cognisant of the whole range of risks.
I also don't care if you say your FB/Insta/Twitter is private. Social networking sites are porous. Facebook in particular is never private, and the data you share will never be expunged.
I think it's not your place, and it's not your right.