CHILDISH THINGS by Valery Larbaud, translated from the French by Catherine Wald (Sun & Moon: $13.95; 201 pp., paperback original). Wald's sensitive translation offers English readers their first opportunity to enjoy these evocative stories that were first published in 1918. The narrators are upper middle-class boys and girls learning their first hard lessons about class distinctions, parental expectations and romance. In "Summer Homework," an impatient, slightly confused boy complains, "In the funny papers you see jokes about 'enfants terribles;' why were there never any jokes about terrible parents? Maybe it's because they're too terrible. But there's another thing: little by little you give up trying to make yourself understood."