The quote comes from "An Open Letter to the American People" New York
Weekly Review, 17 February 1866, 1. The full quotation reads:
"The most useful and interesting letters we get here from home are from
children seven or eight years old. This is petrified truth. Happily they
have got nothing to talk about but home, and neighbours and
family--things their betters think unworthy of transmission thousands of
miles. They write simply and naturally, and without straining for effect.
They tell all they know, and then stop."

Victor Fischer
Mark Twain Project