Where? Citation please. I’m pretty sure it’s both/and, clear from a traditional (i.e. not modern) understanding of Sacred Scripture.
Not really: See
original sin, “in Adam, all have sinned” (possibly a quotation of St. Augustine in that article – or maybe that’s actually St. Paul in the Book of Romans). This is a profound topic, I think involving the reality that we are collectively one body, not only individuals. Thus we must be reborn into Christ, to leave this sinful collective body for a redeemed, clean collective body.
I think you are correct to say that those under the age of reason have not yet incurred
personal sin, but this does not imply innocence (free of all sin).
You seem to be suffering from being raised within the modern hedonistic philosophy that sees pain as the ultimate evil to be avoided. If you will be Christian, you must realize that this is erroneous: The ultimate evil is not pain, but separation from God. I would agree with you if we were discussing God commanding someone to commit sin (which causes this), but the only case we have approaching that is Abraham’s trial with Isaac, and we see upon analysis of this case that God was not commanding him to sin (i.e. willfully commit filicide), but was rather testing his faith/obedience.