Anyone who has ever raised or even dealt with a small child knows that there are two levels to "bad-itude" ... doing wrong out of ignorance, as a little child might, and willful disobedience. One does not punish a three-year-old for doing what he had no clue was a bad thing; one merely stops him/her and tells him/her that it's bad, not to do it again.
One of our Lord's instructions is this:
Matthew 5:48Be ye therefore perfect, even as your Father which is in heaven is perfect.
There is not a one of us who can say that he or she is as perfect as God the Father.
As to Mary, I personally feel that there are two ways to go wrong, and both involve eisegesis. Catholics, out of respect for our Lord's mother, tend to read everything in a manner which holds her up as ideal. (And they do have a point, on a few things: His addressing her as "woman" was not chiding or disrespectful in the language of the day, though to us it sounds that way.) But on the other hand, we are not supposed to be judging her. Reading into "a prophet is not without honor" the implication that Jesus was condemning his own home life is quite literally reading things into the text that may or may not be there. In any case, the Catholic teaching is that Mary, like the rest of us, needed a Savior; it was merely that, in their understanding, His grace worked "ahead of time" to preserve her sinless, in order that Christ's human body and soul be conceived, carried, and born in a sinless way and from a sinless source.
We need to trust more in God's mercy, and less in our own efforts to confine His grace into the straitjackets of our theology. Yes, Paul said "all." And we have evidence here that children believe. Nobody expects a small child to obtain a doctorate in theology. They believe in proportion to their capacity, and their belief and trust is simple and pure. What does a baby know of its baptism? Who can say? God knows, and He is merciful.
Pope Benedict XVI recently stirred up controversy by suggesting that God's grace may be sufficient for those who die too young to believe, for those who were never aware of baptism, etc. Suppose we humbly accept that we, who all definitely are sinners, are saved by His grace alone, and that He is quite competent to know the hearts of the proverbial 3000 BC tribesman, the guy in the jungle who hasn't heard the Gospel, the child who dies young, and all the rest of the "well, what about..." examples raised to question people's theology.
We know Who saved us, and we have our marching orders from Him. And that includes trusting in Him, and working for Him to teach and to bring peopel to Him.