My feeling on this is: forget Harry, why didn't Michael tell his kids this stuff? There's no way a Knight of the Cross isn't informed on this subject, or the consequences of messing around with dark magic. It wasn't Harry's responsibility to educate the children of a friend he didn't talk to much anymore, especially given that their mother didn't want him near them. Both Molly's parents could and should have done it, even with, maybe especially with, Charity's valid issues on the topic.