Seems to me that your question isn’t so much an issue with OSAS as it is with the idea of Original Sin itself, which is common to all christians. If we’re all tainted by sin at birth, then we ALL have to wonder what happens to children that have NOT had an experience of sanctifying grace (baptism in catholic theology, the sinner’s prayer in evangelical protestant).
Catholicism has no dogma on precisely what happens to unbaptized infants who die. The theory of limbo was most popular for a long time, but seems to have drifted aside these days in favor of the idea of God somehow offering these people some other extraordinary means of Choosing God or not of which we have no knowledge. Thus they may have the same chance at heaven we do. Bottom line: we haven’t been told.
Protestants can’t really be expected to have any BETTER explanation since our own understanding is so limited. But the non-sacramental protestants DO have a larger scale of problem since their pool of potential limbo candidates is much larger than ours (they believe salvation can’t happen before the age of reason). I suppose they too could fall back on the possibility of Grace offered in a way of which we have no knowledge.
