At the healing of the man born blind, Jesus told his opponents that if they were blind they wouldn’t be accountable, but since they say “we see” that their sin remains.
Sounds quite a bit like P-Marlowe’s take on this.
So much more for children. They don’t “see”, so their accountability is absent.
A better way, IMO, to permit us what seems like a sensible compassion for infants is that God's mercy can extend to whomever He wants -- the mentally-handicapped, the bushman in Africa, the infant dying so young. We can't forget that it is God who determines their lives; who sets their boundaries; who gives them exactly the number of days He wants them to live.
And within God's perfect scenario for His entire creation there is, we can confidently hope, the real possibility for His grace to extend to those who seem so far removed from Him.
We simply don't know for sure. But this is one more good reason to believe in the covenant family, ordained by God for His glory. That fact should motivate Christian parents to raise their children in a faithful household so that they learn to kneel to none but Christ.