A Jew's life under Nazis; Gideon, by Chester Aaron. New York: J. B. Lippincott. 181 pp. $10.50. (Ages 12 and up.)

Written as a letter to his wife and children in 1978, Gideon explains to them a past that had to be told - his own experience, beginning as a 14-year-old Jewish boy and including his survival in the Warsaw ghetto of 1942-45.

During these wartime years, Gideon passes as a Pole and becomes involved in smuggling goods in and out of the ghetto. After his father's death at the hands of the Germans, he directs all his energies toward helping the ghetto's resistance movement.

Gideon's hope is that by telling his story he will give his own children a sense of what really went on in the ghetto - the day-to day reality of it - so they will not view it simply as ''ancient history.''

The author, through Gideon, succeeds in doing just that.