Once taught to me long ago
D&C 88:38 Every spirit of man was innocent in the beginning; and God having redeemed man from the fall, men became again, in their infant state, innocent before God.
Reading this verse as saying that it says:
1) "Every spirit of man was innocent in the beginning"
2) "God having redeemed men from the fall"
as a consequence
3) "men became again, in their infant state, innocent before God"
Those last two mean that it was because of the redemption from the fall that men become innocent before God when they enter their infant state. They also say that this is a return to innocence, i.e. it says we become innocent "again" when we are born into mortality.
Put differently, they say that our spirits not innocent before birth. Our spirits were once completely pure and innocent, but that changed as we misbehaved. However, because of the redemption from the fall, when we are born into this world there is a cleansing akin to baptism, and all our former misdeeds are washed away, making us innocent before God once again.
Note that this isn't just a statement that we forget everything at birth and become helpless. It specifically says that the innocence we return to at birth happens only because God redeemed men from the fall. Thus it is not just forgetfulness, but cleanliness that is at issue.