And Scripture also speaks of children not having done good or evil before they were born, (Romans 9:11) and of a times when a "child shall know to refuse the evil" (Isaiah 7:16) thus such cannot be culpable for sin before that time. Meaning "all have sinned" refers to those who are accountable beings, not infants at conception or before they are able to know right from wrong and choose btwn the two.
But at least here you are attempting to deal with a doctrinal issue based on what Scripture says, rather than appeal to mysticism in lieu of Scriptural substantiation.
IIRC; MormonISM has a slightly different take on this...
https://www.lds.org/topics/spirit-children-of-heavenly-parents?lang=eng
(And we think Rome's followers are deluded!!)