Time Travel

I've always been interested in time travel stories. I recently watched Kate and Leopold, which has the premise of "meant to be" time travel - it doesn't affect history because the person already entered the past, even though they haven't left the future yet. The "portal" time travel device employed is one of my favorites so far.

After that I read a Christian time-travel novel I've had sitting around for a while, Twice upon a Time. A man travels to past for about a minute and his friend stays. When the man goes home, he finds out his son drowned four years ago, though the man saw him alive just yesterday. No one else in the world knows anything happened, and the man has to go back and figure out what his friend did that changed history. He's the only one who knows that his son's alive since he went back for that one minute. It's quite complicated.

But in the young adult time travel novels I've read, there's very little about changing history. The most mentioned is an object brought from the future and accidentally shown to someone. Is this because young adults can't grasp the concepts? Or aren't the books long enough to go into this?

Perhaps time travel changing history is only a plot device used when need, ignored when not.