I've been disappointed in the kid-oriented nature stuff. My nature loving 5 y/o DS1 and I have found some clips on YouTube (a friend suggested searching for Attenborough to find nature show clips). Great for a kid's attention span, but without dumbing down the content.

We also found a show on PBS that we loved, it was a 1/2 hour Nat'l Geographic (I think) show that surveyed several topics. I can't remember the show title now.
At any rate, I'd highly recommend PBS nature shows - I grew up on National Geographic and Nova and they really fed my love of nature (well, paired with lots of playing outside and camping and such!).