Perhaps we should look at this another way.
Since, according to Mass law, parents CAN be held criminally responsible for their child's activities until that child reaches 18, a defense can be made that it is then those self same parents' right to make sure that crimes are not being committed by that child.
Since the child's penalty can be forcibly assumed by the parents (to pay a fine and, in some cases, lose that child for, say, not stopping a sexual relationship with a pedophile), courts have given parents a great deal of latitude in investigations in their own home.
Parents have been invading the privacy of children for years. This is not legally different from reading a diary or searching a child's room for pot. I know of not one case where a parent was successfully sued for such an invasion.
I do, however, know of several hundred cases where parents were disciplined for NOT knowing what was happening in their own home concerning their minor children.
Therefore, the legal precedent is clear... parents must, under penalty, know what is going on.
Now, before you tell me how the French, Germans or Laotians think of this, realize that such things are not legally binding in this country.
Here is a little research job for you... you seem to belittle anyone that doesn't do their homework. Find me one case where the United Nation's Declaration of Human Rights preceded ANY law in this country.. in case you were asleep in civics class, the Constitution is the supreme law of the land... I see no documentation or court record showing that supremacy has been revoked.
Until you can show differently, let's just keep our thoughts around relevant law, okay?
Oh, and parents can also legally restrict their minor child's association with anyone they deem appropriate... even if it offends your delicate sensibilities - or even mine, for that matter.