I am all for the fact that God can not do an unjust thing.
However, I do not believe that God would be unjust if He allowed an infant who died in infancy go to hell.
I don't want to read too much into what people like you mean when they say "that God can not do an unjust thing".
But it sure seems like they are saying that God would be unjust if He allowed an infant to go to hell.

I don't think it is wrong to have the hope that an infant who dies in infancy is with the Lord, but I don't think we can know either way. In fact that is where my sentiments lie.

Tom