It entirely depends on the kid. Nova did a series on the formation and geological history of North America, and the local station happened to air it following

It entirely depends on the kid. Nova did a series on the formation and geological history of North America, and the local station happened to air it following their Sunday morning children's programming. My six year old sat through each episode entirely enraptured with it, and he asked all sorts of questions after. My younger kid, not so much. I know Cosmos is going to be above his ability to completely understand, but I think it'd probably keep his attention and prompt questions.