Good topic, Decem!
I think it's about providence, not predestination. Like St. Alphonsus says (Reply #3). I think, his first answer is key, although he deems the latter "more correct".
Assuming, I'm not in the state of grace. Even if God wills that I repent my sins, he may allow that I die right now before even having a chance to do so.
How?
He knows by providence in advance whether I will or I won't repent. I case I won't, there is no reason to wait and give me a chance.
Same thing with unbaptized children who die before the age of reason. God knows in advance that in case they'd live on, they'd later reject Him, anyway.
An aside: This puts me on the same foot with unbaptized children. With respect to the question, I am not privileged, just because I'm baptized. The unbaptized at least is in limbo. If I'm cut off, because God knows in advance that I won't make it to heaven anyway, then I'll go to hell, below limbo. And if I live to be 90 or 110, still the same result.