Levertov seems to be describing the "discovery" of a secret which, like so many others, is found by the naïve mind of a child. The secret is found in a "sudden line of poetry." It is not a painfully thought out concept like that of a politician or a philosopher, it is realized in an instant, it is an epiphany found by two girls.
The author goes on to tell us that, although she wrote the line in one of her poems, she does not know what the secret is, and the girls have no intention of telling her. In fact, they have probably already forgotten it. This reminds me of all the "secret" things I had when I was young with my friends. We organized secret clubs, we found secret treasures, we had secret places to play; we never had the secret of life, though. And most of it ended up being forgotten eventually; fading behind all the more important experiences we had over the years. Yet it was our simplicity, our belief that we had something which no one else in the world had, something special, that made it all so “secret” that we never told anyone about it.
“I love them for finding what I can’t find.” The girls have found a miracle in one of the speaker’s poems. They have found something that unintentionally became momentarily life altering. And the speaker loves them for forgetting it, because that means the girls can find it over and over again in different places and occasions for the rest of their lives, perhaps saving some of their innocence.
The last thing that Levertov writes is that she loves the girls most of all for assuming that there was this secret of life, and for wanting to know it. She loves the girls for their hope and ambition to find something worthwhile in, or about, life. And, as a whole, I don’t think there is any secret meaning to this poem, or any deep dark implications of death or loss. To me, this is a poem about the optimism of the author when she sees the potential in the dreams and desires of a future generation, and the innocence of said generation.