What lydz said is correct. Jehovah can read anyone's heart (King David said Jehovah knew him in his mother's womb; Jehovah knew Esau would not appreciate his birthright, etc.)and he knows what kind of attitude or condition someone is predisposed to even before they are matured. As lydz said, if their parents continued in an unrighteous course, even after they were preached to for FORTY YEARS (? is that # correct?), it was very unlikely that they wouldn't follow.
But sometimes (even after explanation)when I can't seem to see the justice in something, i remember Jehovah is always perfectly just and righteous in EVERYTHING. Even when things don't seem just in our minds it's b/c our sense of justice and wisdom are very imperfect and we cannot see the whole picture.
oh, and yeah, that's another thing - how in the world was it possible for noah to like preach to everybody on earth
you have to remember the the WHOLE physical earth wasn't populated yet, it was still fairly close to the beginning of human history. no one spread out THAT far yet. plus it could be likely many people heard of the ark and came to see it. People talked and they mocked. If they were honesthearted they could have went up to noah and asked what it was all about.