unbidden

今日取り上げる単語 "unbidden" もThe Case against Perfectionに何度も出てきます。
We choose our friends and spouses at least partly on the basis of qualities we find attractive. But we do not choose our children. Their qualities are unpredictable, and even the most conscientious parents cannot be held wholly responsible for the kind of child they have. That is why parenthood, more than other human relationships, teaches what the theologian William F. May calls an "openness to the unbidden."
友人や配偶者と違って子供を選ぶ事はできないと言っているので "unbidden" は望んでいなかった子供の様に思えますがどうでしょう。 "unbidden" が出てくる個所をもう一つ引用します。　
The problem lies in the hubris of the designing parents, in their drive to master the mystery of birth. Even if this disposition does not make parents tyrants to their children, it disfigures the relation between parent and child, and deprives the parent of the humility and enlarge human sympathies that an openness to the unbidden can cultivate.
推測は当たっているようですが、辞書で "unbidden" の意味を確認します。
・American Heritage Dictionary: Not invited, asked, or requested; unasked: unbidden guests; comments unbid and unwelcome.
・Cambridge Idioms Dictionary: not invited or wanted: At night images would come unbidden into her mind.
望んでいなかった子供と言うのは、ここでは、子供の知能、体力などの能力が望んでいるレベルではなかったと言う意味で使っているようです。