If you are curious about the long-standing Catholic debate on this subject, I'd recommend googling "limbo"... Seriously crazy stuff.

I believe it is something the original authors forgot to cover, and the church had to cover their ass later given the infant mortality rate throughout history.

Reminds me of a really old joke:

Eskimo: 'If I did not know about God and sin, would I go to hell?'

Priest: 'No, not if you did not know.'

Eskimo: 'Then why did you tell me?'

It's called "the age of accountability" and it is a totally invented doctrine and there is no basis for it in scripture. It is inferred / argued from principles, not from holy writ. One of the principles is that salvation is an intentional decision and thus requires a certain capability to think abstractly in order to be responsible for that decision, hence, god in his grace would not judge an innocent (a child, a developmentally disabled person, etc).

It just reflects the practical problem that you have to say something to deflect from the notion that technically, an unsaved person who dies is an unsaved person who dies (and goes to hell) and the fact they are a child has nothing to do with it so far as scripture is concerned. No pastor wants to deal with the fallout of telling a devastated parent that their infant is going to hell.

Some of this reflects the fact that the whole concept of childhood as a safe, fun, nurtured, carefree time is quite new on the human stage, it was only a fairly fully formed concept just a century ago; before that, child labor was common, for example, and compulsory free education was not the norm. And attitudes towards the developmentally challenged were shot through with much fear and loathing even more recently than that. The need to be tender and sympathetic to children and the disabled means the harsh dogma of Christianity must be softened in this regard.