"Mundus et Infans" means the world and the child. On one level, Auden's poem is simply a tongue-in-cheek description of the way a baby, with all its needs and demands, naturally tyrannizes an ordinary household.
Auden uses humor as he compares the infant to an "ogre" and "dictating" being whose goal it is to "seize supreme power." The baby is described by the speaker as "this beast / [that] Dares to exist without shame." It is ironic or unexpected that something as tiny, cute, and helpless as a baby can wield such a will to power.
The speaker's dryly ironic, humorous tone is underscored by the prosaic style of the poem. Although Auden slips in rhyme, assonance, and alliteration, all lending a sense of rhythm, the poem has the conversational feel of a piece of prose.
Beneath the humor of the baby's behavior is a political commentary. The poem was written in late 1942, at the height of World War II. Much of the description of the baby's behavior also would be instantly recognizable as the behavior of a tyrant like Adolph Hitler. He, too, had the goal of seizing "supreme power" and was seen by many as a "beast ... without shame."
Auden's poem suggests that there is something innate in human nature, from the time of birth and before (the poem opens with the baby "kicking his mother" in the womb to be born) that is dictatorial and monstrous. Although we are reassured that the baby "has not yet gone mad," as Hitler has, and that "we were no worse at this age," the speaker also hopes that this baby never gets too much power, that he never becomes an "Important Personage," because buried in every person are the seeds of a destructive force.
In comparing an ordinary baby to a tyrant like Hitler, the poem shows that Hitler is not a one-off or an anomaly, a special, particularly evil species of humanity. We must all be on guard, for anyone, any time, has the capacity to turn to evil.