Though Paul is making a generalization of all humanity, we would agree that there are exceptions. For example, a child below the age of reason is not capable of committing actual sin. By definition he can’t sin, since sinning requires the ability to reason and the ability to intend to sin. This is indicated by Paul later in the epistle to the Romans when he speaks of the time when Jacob and Esau were unborn babies as a time when they "had done nothing either good or bad" (Rom. 9:11).
Jesus is another significant exception to the rule, having been exempt from actual and original sin (Heb. 4:15). If Paul’s statement in Romans 3 includes an exception for the new Adam (Jesus), one may argue that an exception for the new Eve (Mary) can also be made.
I would not so agree. Sin isn't something we do - it's something we ARE, until regenerated by God's unmerited favor. So, although a child may not be capable of committing sin in the sense of making a conscious, rational decision, the child is still sinful by nature.
Any other belief begs questions that require some serious theological gymnastics to get around.