I looked for exactly this when my daughter was six, because I had very little faith that her public school was going to cover it in enough depth. Nothing I found really hit it out of hte ballpark, but these were worht looking at:
Life on Earth: The Story of Evolution -- stunning torn-paper collage illustrations. It's worth buying just for that! I thought the text was both a little too general AND a little too technical, and it doesn't cover human evolution.
Magic School Bus Inside the Earth -- decent survey of geological time, because they drill down through the layers of the earth's crust and mantle.
DK has an evolution book in their Eyewitness series -- I haven't looked at it and I personally find their style too cluttered and fragmented, but they usually do a great job with accurate presentation of science for kids. They have an Early Humans one too.
This one is brand-new, just published this month:
http://www.amazon.com/Evolution-How-..._edpp_ttl_in_f
I haven't seen it in person, but it looks promising!