In 2015 the population of the US included ~73,783,981 children. There were 1,093 murders of children in that year. The poem would perhaps be more accurate to say:

For every 67,506 loved children, there is a child broken, bagged,
sunk in a lake.

When the poem says the world is “at least half terrible”, it would perhaps be more accurate to say that it is at least 0.0014% terrible.

I don’t have any data for violence against birds.

The poem argues that “life is short”; that parents have sometime been reckless; that evil is done, to us and to others, such that love and safety is luck, and a narrow luck at that; that a parent will, and perhaps must, keep this wisdom from their children; that the world, in truth, must be sold, that it is redeemed only in its telling; and that those who know life’s travails have some responsibility to reassure the naive despite them, even if that is insincere.

But what the poem, in its rhetorical trimness, is trying to convince us of really is not the world’s “good bones”, but the bad against which these are supposedly redeeming. We imagine the persuasive act in the poem is that discussed, the necessary ‘selling of the world’ to one’s children, when in fact it has already happened, and to us: the cartoon diptych of good and evil which, in confidence, we are assumed to agree.

The speaker smirks at their “delicious” past indiscretions, and we suspect that, even in their pessimism, the speaker emphasises the world’s evil to aggrandise their knowingness rather than to bring realism to their message to their charges, in a lecture that they rehearse to us but still ‘keep from their children’. Like “any decent realtor”, their interest is in the persuasive drama of the house’s sale, in their commission and not in the broken boiler and the slow years of rising damp. We suspect that what they tell us, in poetic confidence, is not that they are keeping things from their children, but that they are not, after all, entirely serious.