Each person and each group should and do have the right to form their own system of interactions, whether that is religious, political (or lack of) or anything else, providing they do it at will.
When a child gets brainwashed by their parents, it is against their will.
Indeed, one's own thinking system is one's own right too. Brainwashing is essentially the imposing (injecting) one's thinking inside another's without his will. (Like when you're put by force to memorize a book and then repeat it.) Or the prohibiting one from thinking his own thoughts.