That is like the USPS opening every letter and scanning what is inside. Even if it's done with machine learning, that is absolutely not okay, no matter what it is for.
But I understand your concern here. All our lives are now digitalised and maybe stored forever somewhere and anything you do, even if completely benign, could be considered a crime in some country or even in your own country in a few years from now.
(Not trying to make a counter-argument to your comment. I genuinely don't get this part).
I'm not sure how the image is displayed on the parents' device - it could be sent from the child's device, or the image used to compute the hash that the child's image matched could be displayed.
That is different from searching them for illegal material based on what illegal implies at a given time and location.
Don't think the US is taking a hard-line stance here. If it were possible to non-destructively scan mail for CP they absolutely would.
If i recall correctly isn’t there a legal distinction involved in mail privacy, I recall an explanation involving the difference between postcards and letters with respect to lawful interception…
"The Fourth Amendment protects the privacy of U.S. mail as if it had been kept by the sender in his home. Ex parte Jackson, 96 U.S. 727, 732-733, 735 (1878); U.S. v. Van Leeuwen, 397 U.S. 249, 252-52 (1970). It cannot be searched without a search warrant based on probable cause."