Ah, educating the young. A task made both easier and harder in the Age of (Mis)Information.
A book that I grew up with is
Life Story, by Virginia Lee Burton. It's a bit dated (was published in the early '50s), but is decidedly secular in its approach. Beautifully illustrated (Burton is a painter), it does a good job of showing the immensity of time and the wonder of discovering knowledge.
Another author is Holling Clancy Holling, in particular his books
Minn of the Mississippi and
Pagoo. Both of those are more about natural history than anything else, but also touch on evolution, geology, biology and history; showing the connections between different branches of science.
You might try the "grains of sand on a beach" being a year each, or "stars in the sky" on a clear night as a year each.
Excellent idea.