God damn it, you’ve got to be kind.
I think most nonreligious parents would really enjoy the first two chapters of The Autobiography of Bertrand Russell (“Childhood” and “Adolescence”). Among other things, this section recounts his transition from a Christian upbringing to agnostic atheism.

Bertrand Russell portrait by Memnoch-Plopk, Deviantart
But another passage much later in the book would have been worth reading the whole thing just to find:
Ever since puberty I have believed in the value of two things: kindness and clear thinking. At first these two remained more or less distinct; when I felt triumphant I believed most in clear thinking, and in the opposite mood I believed most in kindness. (vol 2, p. 232)
Nonreligious folks are not unkind. Many are the gentlest and kindest people I know. But in our meetings and conferences and blogs and social media, we sometimes overlook the topic of human emotional needs. We focus instead on the need for clear thinking — until we are feeling “the opposite of triumphant” and find ourselves, as individuals, hoping for a kind word or thought or deed. Russell’s first value rushes in.
As a parent, I find myself more upset by the unkindnesses my children do than by any fuzziness of thought. And I find it harder to forgive my own lapses in the former than in the latter.
Kurt Vonnegut circled around the same idea in God Bless You, Mr. Rosewater. The protagonist is asked to say a few words for the baptism of his neighbor’s twins. What do you say to welcome new lives into the world? Here’s what Vonnegut found fitting:
Hello, babies. Welcome to Earth. It’s hot in the summer and cold in the winter. It’s round and wet and crowded. At the outside, babies, you’ve got about a hundred years here. There’s only one rule that I know of, babies—God damn it, you’ve got to be kind.