Children learn what they experience. They are like wet cement. Any word that falls on them makes an impact.

Haim Ginott (1922-1973) Israeli-American school teacher, child psychologist, psychotherapist [b. Haim Ginzburg]

Between Parent and Child: Revised and Updated Edition, ch. 10 “Summing Up” (2003 ed.) [with A. Ginott and H. W. Goddard]
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Frequently paraphrased (e.g.) as "Children are like wet cement. Whatever falls on them makes an impression."

This is usually cited as being from the original 1965 edition of the book, but cannot be found there. Instead, it appears to be from the 2003 edition, as revised and updated by his wife, Dr Alice Ginott, and Dr H Wallace Goddard. It is unclear if Haim Ginott may have used this phrase in other contexts.