An encouraging, undogmatic book which should help parents feel less awkward in dealing with those inevitable and sometimes embarrassing questions which will probably begin when the child is three or four and just starting to compare his/her anatomy with that of mommy and daddy. Dr. Seizer, a child psychiatrist at Boston's Beth Israel Hospital and adviser to sex education programs in several schools in the vicinity, stresses that parental responses must be attuned to the child's cognitive level: too much information can be as confusing and frightening as too little. For example, children under six or seven aren't able to understand about intercourse, sperm and fertilization. She includes some ""typical"" questions from pre-schoolers, the six-to-ten age group (latency) and young people. They range from the small child's querie re the penis and why mommy's tummy is so big to the adolescent's sometimes challenging demands to know what mother thinks about ""making out,"" unwed girls with babies, or homosexual couples. She reminds everyone that clarity, matter-of-factness and a willingness to listen encourage a frank discussion and further questions. Tolerant without being fashionably permissive, Selzer notes that the old sanctions against teen-age sex are no longer as valid as they used to be, but adds that; in her opinion, intercourse at an early age ""is harmful emotionally to both boys and girls."" Within a generally Freudian developmental context, this is sound, middle-of-the-road advice.