If Adam and Eve didn't know good from evil, how can we say they sinned (according to Roman Catholicism)? When God made Adam and Eve he said they could eat anything except from the tree of knowledge of good and evil. It sounds like that tree grants the knowledge, so before they ate they didn't know good from bad.

But I was taught by Roman Catholics that they sinned. Doesn't sin require intention? If a young child takes something that is not his, we don't say he sinned, right?  Because he didn't know better. Or is that wrong and we do say he sinned?

Yes they disobeyed God but why should they know that's bad?  I want to understand why they (and everybody after them) were punished for this.  According to Roman Catholicism: if they did know that what they were doing was bad, how did they know, or if they didn't know, then what makes it a sin?
Your question appears to ignore some important facts that need to be taken into account.


Evil existed before Adam and Eve ate from the Tree.


Rev 12:7  And there was war in heaven: Michael and his angels fought against the dragon; and the dragon fought and his angels,


The act of eating the fruit was not the beginning of evil that it seems that would 
appear to be the rebellion of Lucifer in Heaven and the war that resulted from it.



Sin in all of it's forms actually boils down to disobedience of God.


Therefore we can accurately say that Adam and Eve sinned in the sight of God.



I must disagree with your concept that they and their posterity are being punished for having the knowledge of good and evil.


They and we are held guilty of choosing evil over good and thus disobeying God, 
Without the knowledge of good and Evil such a choice would not be available.

Your example of the child taking something that is not his is a prime example of my point.
Both fault and punishment would be contingent on whether or not the child knew that taking the object was wrong. Most assuredly we would not punish a baby for taking a rattle from another baby, but we would not hesitate to punish a teenager who stole a CD from a store.
This Godly precept is the basis for all of our laws in the United States, that being that at a particular point in one's life they are aware that their actions are wrong.
We might rightly say that the command not to eat the fruit of the tree of the knowledge of good and evil, was the first law given to man by God.

Genesis 2:17  But of the tree of the knowledge of good and evil, thou shalt not eat of it: for in the day that thou eatest thereof thou shalt surely die.

This command also includes the punishment for disobedience.
You may say that the Serpent beguiled Eve and that should somehow mitigate the severity of he disobedience, but we need also remember that Adam was not, and willfully disobeyed God.
That fact is also our downfall in that we willfully choose evil whether it be embracing the wiles of the world, or rejection of salvation. We each have the ability to either choose an eternity with God, or an eternity in the lake of fire.