The "one hundred children" theme paintings were considered auspicious because more children meant a flourishing family. The number one hundred is taken metaphorically to mean "innumerable," although many artists took pride in their ability to depict the full complement of subjects.  In the One Hundred Children Playing painting, the children, all boys, are shown engaged in a wide range of pursuits, from those typical of childhood, such as flying kites and sliding down a stairway railing, to activities meant to emulate the adult males around them - playing the zither, writing poetry, and enacting a ritual washing of the Buddha.  


Below is another example of this theme, but without the attempt to portray a full hundred children.


Su Hanchen, attrib. (active 1130s-1160s), Children Playing in a Palace Garden