A unique horror show airs at 8 p.m. today (Thursday) on the Mobil Showcase Network (check local listings for stations, mostly independent). ''The Children's Story'' is a classroom parable about the vulnerability of children and how a shrewd government can indoctrinate innocent intellects with distorted points of view.
It is a half-hour dramatization of James Clavell's ''The Children's Story,'' which first appeared in the Ladies' Home Journal and more recently in book form (New York: Delacorte Press, 1981). According to narrator Peter Ustinov it happens when ''They (a foreign power) just conquered us.''
A new ''teacher'' distorts the pledge of allegiance, the value of prayer, the basic morality of democracy with just a few ''simple'' tricks.
The teacher is acted with slightly stilted skill by Michaela Ross (who happens to be Clavell's daughter) and a group of charmingly earnest youngsters.
This slight but important half-hour in class is a shocking lesson about the sometimes forgotten power over our children which we delegate to our schools. It is also an unnerving lesson in the danger of the hidden message.