I think it seems like an interesting question, if slightly insensitively asked. It also ties in with a question I've been quite interested in for a number of years - what about those who are mentally ill such that their perception of reality is horribly twisted? So here's a few quick thoughts from me...
1. We can and should trust God
We don't know everything. We don't know the state of people's hearts. God does. He has shown in Jesus that he is incredibly loving, even to us, when we reject and ignore him. We can trust him to be fair or better and to do what is right, far more so than we could trust ourselves in the same situation.
2. The Christian faith is not a set of propositions
One thing I dislike is the responsive form of the creed that we use sometimes in church. Here's a sample...
Q: Do you believe and trust in God?
A: I believe in God, the Father Almighty...
The question asks "believe and trust"; the answer is just "believe". We so often seem to reduce Christianity to believing the right things, rather than acting on them. The creeds were originally written in Latin, and the word that translates as "believe" is "credo", from which we also get "credit". It is much closer to "believe and trust" than just "believe". And in a strong way, it's the trusting bit that really matters. I think, for example, of the woman Jesus met who had haemorrhagic bleeding:
And a woman was there who had been subject to bleeding for twelve years, but no one could heal her. She came up behind him and touched the edge of his cloak, and immediately her bleeding stopped.
"Who touched me?" Jesus asked.
When they all denied it, Peter said, "Master, the people are crowding and pressing against you."
But Jesus said, "Someone touched me; I know that power has gone out from me."
Then the woman, seeing that she could not go unnoticed, came trembling and fell at his feet. In the presence of all the people, she told why she had touched him and how she had been instantly healed. Then he said to her, "Daughter, your faith has healed you. Go in peace."Luke 8:43-48 (NIV)
The woman in question doesn't seem to have had a great intellectual understanding of the truths of the gospel. Her "faith" seems to have been little more than an almost superstitious trust that touching Jesus would help. Yet that "faith" enabled her to be healed. It doesn't take someone who understands and can explain Calvin's Institutes or Aquinas's Summa Theologica to have faith in Jesus. It just requires someone who can trust him, regardless of their intellectual capacity. And pretty much everyone I know is capable of trust.
Of course, trust in Jesus produces a change in lifestyle, and the extent of the change will of course depend on circumstances including intellectual capacity...
3. Difficult questions
What about stillborn children / aborted fetuses? Does the status of the parents matter in those cases? What about people who have been in a permanent vegetative state for a very long time? What about those who are severely delusional?
I could hazard some guesses to some of those, but that's all they'd be. I don't think I will know all the answers in this life either, but I'm happy and content with trusting Godmy Saviour, my Rock and my Redeemer.