That is *not* true. Children learn the basics at a very early age.
You are saying that if a toddler knocks the gear shift out of place and the family car runs over his mother, that the toddler sinned, but it won't be held against him. That's just wrong.
A rather unfortunate example, as there is no intent in the action. It would not be a murder in that case, but a mere accident.
A better example would be one of covetousness:
A child purposefully takes a toy from the home of a playmate because he wants it for his own. That is sin, and he knows it to be so, as when he is caught at it he exhibits guilt.
Toddlers don't understand and their actions are not sinful. Sin is about the will, not the actions.
The example above proves my case, and it is from within my own house. The child I speak of was only 2 1/2 years old when he took my child's toy. When his parents forced it's return, he was fully guilt ridden and apologetic for his actions.
That instance holds all the prerequisites of sin. Children hold all the willfulness of their parents, but without the good sense of experience.
A child is born and one minute later dies. Has this child "learned" right from wrong? Has this child committed any sin?
If he knows it to be so (truly knows it and not just knows that Mom and Dad will be mad) then he is not the child we are talking about.
It is for this reason that I used the word (I try t carefully pick words) "infant."
If infants have no capacity for knowing and choosing and understanding the remifications, then they do not sin.
And the "all have sinned" Bible verse must allow exceptions and can not be used as a bludgedon to deny Mary her role.
That was the point.