My daughter, now 3.5 yrs has been interested in adult shows since she was 2 or 2.5. Currently she watches things like Bill Nye which she loves, Nova etc... She will sit and watch long ones with me about Einstein, quantum mechanics, animals, Earth's forces etc and ask questions about them. Of course she doesn't understand quantum mechanics, but the fact that she sits and watches them and understands enough to ask questions about it is pretty cool.
We have always given her the opportunity to watch kids videos, which she does the majority of the time, but sometimes she requests certain documentaries.
At 3, she walked up to me while I was sitting at the computer and said, "Dad, I love the aurora borealis!"
First I thought, wow how did she pronounce that so well, and then I asked her, "Well, what is the aurora borealis."
She said, "The aurora borealis is the colors in the sky, they are beautiful....God made them."
Then of course I pulled out some of my space photography books and we looked at all kids of stuff for the next 30 minutes or so.
I say let your kids watch the History Channel or whatever, it can really only enhace their learning. Yes, I feel that violence is not really something that children should watch, but in an educational context and not just horror or action movie violence, it is not so bad.
Here is something I have been wondering about...my daughter has a firm grasp of death and finality. She knows what can kill someone, illnesses that cause death and other things like that, is this normal?