A Wrinkle in Time

From the text:

Even Charles Wallace, the "dumb baby brother," who had an uncanny way of knowing when she was awake and unhappy, and who would come, so many nights, tiptoeing up the attic stairs to her—even Charles Wallace was asleep.

He looked very small and vulnerable sitting there alone in the big old-fashioned kitchen, a blond little boy in faded blue Dr. Dentons.....

It was true that Charles Wallace seldom spoke when anybody was around, so that many people thought he'd never learned to talk.