''THEIR pictures are usually rather badly composed, but their realism gives them a compensating charm,'' observes the anonymous author of the entry for ''Le Nain'' in the Phaidon Encyclopedia of Art and Artists. But ''Peasant Children,'' shown here, surely refutes such dismissive analysis.
The reference is to ''they'' because there were three Le Nain brothers, Antoine, Louis, and Mathieu. Born in Laon, France, they made their name in Paris during the first half of the 17th century. They painted portraits, religious paintings, and most notably what later came to be known as ''genre'' subjects. ''Peasant Children,'' which is painted in oil on copper and is less than a foot wide by 8 1/2 inches high, is very characteristic of this side of their art.
Not many of their paintings are signed, and those that are have no initials. So there is no certainty about the contribution of each brother, and, though theories abound, the separate individualities and abilities of the three are still conjectural.
The Burrell Collection in Glasgow, to which this painting belongs, attributes it to ''Antoine Le Nain'' - probably because he, the eldest brother, was described by a contemporary as excelling in ''miniatures and portraits in small.'' But the full-scale exhibition of ''Les Freres Le Nain,'' held in Paris in 1978-79, boldly refused to give any of the displayed paintings to one brother or another: They were all ''Le Nain,'' as they themselves apparently wished. Since Mathieu, the youngest, outlived the other two, some paintings must be by him alone - yet there is still no hard-and-fast evidence as to which they are.
All this could seem academic if it weren't for the possibility that an appreciation of the Le Nains' art might be helped by the knowledge that many of their pictures seem likely to have been collaborative. This could explain the particular quality criticized by the writer quoted above as bad composition.
''Composition'' is, anyway, an academic notion, and perhaps this critic is setting standards that are actually irrelevant to the needs and aims of the Le Nain brothers. Their basically simple, quietly immobile groups of peasants and children would only too easily look pretentious, overdramatized, inauthentic in atmosphere, if they were subjected to more calculated, less straightforward presentation. They might become figures reminiscent of some classical frieze; and then their resignation and artlessness, their ''apparent abstraction and self-absorption'' as Michael Fried has put it, would be lost in artistic self-consciousness.
It is the absence of just this kind of self-consciousness that is such a powerful force in these pictures, and it must partly result from the checks and limits inherent in a collaborative procedure.
It is significant that music is being played in a number of the Le Nains' ''genre'' pictures. Perhaps they are, in fact, allegories of sound. The interrelationship of the distinctly individual figures in them evidences a careful eye for a kind of composition not unlike music - a play of intervals and directions. The figures group as musicians might. They harmonize and contrast in the way different voices or musical instruments do.
Anthony Blunt wrote of (Louis) Le Nain that ''he paints the peasants with complete sympathy, but at the same time he resists the temptation to idealize them.'' It is this rather ''honest'' avoidance of idealism which probably appealed to the proponents of ''realism'' in 19th-century French painting. One can see without difficulty that the air in Le Nain paintings might be breathed without distaste by Courbet. And Manet for a short period looked to them for direct inspiration, probably owing to them his lifelong preference for unfussed - uncomposed - presentation of figures in a painting.
But to describe this kind of straight ''realism'' as having ''charm'' that somehow ''compensates'' for poor composition is surely to miss several points. Charm, even if it is sometimes an aspect of realism, is more often its antithesis. ''Peasant Children'' is arguably not charming at all. Nor, of course , is it completely ''realistic.'' Compared with some of the ebullient crudities of Dutch ''genre'' paintings of the same period, this picture shows very peaceable, tranquil, ideal peasants.
But it is a picture which is undemonstratively objective and seems to penetrate the character of each individual, to understand the separateness and secrecy of their particular thoughts. This is even more remarkable in a picture of children.
If the positive nature of this objectivity isn't appreciated, it could be misread as a lack of coherence or ''composition.'' This little picture, and others (some much larger) by the Le Nain brothers, do not show the dominating coherence of a single artist's vision. But one suspects that if they did, much of their unique quality would be missing.