We must trust in God. Many have posted that children being in hell is so disturbing a notion they can’t imagine a God who would allow such a thing. My sensibilities trend in that direction as well but in the end I figure God knows what he is doing, I trust Him, so I don’t lose much sleep over it.
That trust drives straight to the heart of the culpability question. Again, that really must be rooted in trusting God. Consider Romans 2: 14 - 16:
For when Gentiles who do not have the Law do instinctively the things of the Law, these, not having the Law, are a law to themselves, in that they show the work of the Law written in their hearts, their conscience bearing witness and their thoughts alternately accusing or else defending them, on the day when, according to my gospel, God will judge the secrets of men through Christ Jesus.
That seems to mean God has insights into the conscience, motivations, psycho-baggage, and true culpability beyond anything we pretend to possess. It is a rather terrifying idea, being tried and convicted where our own mental state, conscience, and standards are both the primary witnesses for the prosecution and are seated in the jury box, too!
God knows the truth! He knows it for adults and He knows it for children. He, alone, knows all inner motivations, what all are capable of, incapable of, and subsequently our level of accountability. I trust His judgement!