To put a bit of a crude metaphor on it, if you were to pick a random number uniformly between 0 and 1,000,000, and pre-commit to having a child on iff it’s equal to some value X—from the point of view of the child, the probability that the number was equal to X is 100%.
Conditional P(X|X) = 1.
However, unconditional P(X) = 1⁄1,000,000.
Just like you can still reason about unconditional probability of a fair coin even after observing an outcome of the toss, the child can still reason about unconditional probability of their existence even after observing that they exist. They can notice it’s very low, and therefore be rightfully surprised that they exist at all and update in favor of some hypothesis that would make their unconditional existence more likely.
Conditional P(X|X) = 1.
However, unconditional P(X) = 1⁄1,000,000.
Just like you can still reason about unconditional probability of a fair coin even after observing an outcome of the toss, the child can still reason about unconditional probability of their existence even after observing that they exist. They can notice it’s very low, and therefore be rightfully surprised that they exist at all and update in favor of some hypothesis that would make their unconditional existence more likely.