Looking in

''The Stein children,'' Oskar Kokoschka wrote in his book My Life, ''came before me flushed from playing, clothes rumpled, the girl in a pink and white striped garden frock of the kind that parents seem to think appropriate for the occasion, the boy in the inevitable Sunday sailor suit. . . .''

In a single, defiant, painterly gesture, the Austrian artist managed to shatter most of the conventional and stilted expectations of child portraiture (conventions still vigorously adhered to, for example, in the work of most studio photographers). The brother and sister were not painted to charm or please their parents. They were not prettified or sentimentalized. They were not tidied, hair brushed, socks pulled up. Naturalness more than predominated over arrangement. Nor, at a rather deeper level, were they the innocents of an earlier period. The romantic notion of childhood as a pristine enigma either poignantly lost or to be somehow regained takes an abrupt knock from the picture of the Stein children.

The German nineteenth-century artist Philipp Otto Runge said: ''We must become children again if we wish to achieve the best.'' Kokoschka, on the other hand, seems to indicate that there is something unbelievable about that kind of naive idealism in the face of actual children.

And yet this work of 1909 is not strictly speaking anti-Romantic. It conveys, in fact, with some potency, Kokoschka's basic contention about small children, that they ''are not habituated to life as grown-ups are,'' and that they have secrets which adults just fail to grasp.

''We imagine,'' he wrote, ''that we've made ourselves clear to children when all we've done is to talk down to them. We are no more than vulgar interlopers in their kingdom of dreams - a realm in which, however childishly we behave, we share very little. Yet sometimes in the midst of all our self-deception, we are taken unawares by a moment of sharp regret at ever having become outsiders: it is the moment when a child suddenly looks at us as if we were not there.''

Dr. Stein's little girl looks (even perhaps pulls a face) at the artist who is painting her as if he is invisible. She is no delightful little thing, sweetly accommodated to a charming setting by Renoir. Nor is she one of Gainsborough's entrancing daughters, lightly chasing a butterfly. Indeed both she and her indifferently daydreaming brother, sprawled with ungainly carelessness on the floor (and how accurate Kokoschka is in his observation that children are more natural messing around on the ground than they could possibly have been ''posed'' on some kind of furniture), are given considerable freedom to project their unpretending attitudes onto the picture.

Kokoschka was twenty-three when he made this painting, and he already sees his own childhood relegated to some irretrievable distance; nevertheless he attempts in this painting something approaching a subjective evocation of childhood. There is an attempt to see from the children's point of view. The later extension of this attitude was the deliberate efforts by artists like Picasso, Klee and Dubuffet actually to paint like children - a sophistication ironically incomprehensible to children themselves.

These two ''Children Playing'' belong firmly (whether it is likable or not) to a twentieth-century attitude toward children. To borrow a phrase from an Auden poem, they are ''casual as birds.'' In portraying the boy's momentary quiescence - which might easily turn to boredom if he is restrained much longer - and the girl's open cheekiness, the painter does not present small and unspoiled versions of adulthood in miniature; instead, he touches on something of the urgency and impulsiveness of children, their instinctiveness, their unwillingness to fit a mold dictated for no very persuasive reason by larger relatives - reasons that do not contain their ''kingdom of dreams.''

He has understood to a degree (or is it that he has remembered?) the energy-for-energy's-sake which is so vital and irrepressible in young children. This has its own authentic attractions. And if Kokoschka has abandoned as unrealistic the Romantic conviction of childhood's primal innocence - ''trailing clouds of glory'' - he clearly relishes instead the hint of carefree, primitive rebelliousness that these two children donated, without knowing it, to his art. This is far from being some shockingly cynical approach to childhood. It is a capturing of its quicksilver.