If Adam and Eve didn't know good from evil, how can we say they sinned (according to Roman Catholicism)?

Adam amd Eve "fell ". That is, they fell into mortality, but in the process gained "agency" (free will). This was necessary. If they had not fallen to mortality, they would not have procreated and thus "replenish[ed] the Earth". So the Fall was inevitable and a necessary part of the plan, as was the Redeemer Jesus Christ. Lucifer's plan would have removed Man's agency, forcing everyone to be "good". This would have prevented any learning or progress on our part. Only in a world with good and evil to choose between can we grow to become more like God, and to return to His presence. We have Adam and Eve to thank for that. In their sin we inherit mortality. But because of their choice, and the free will that that brought to us, we have the opportunity to choose the good, to be saved from sin, and to inherit eternal life with God our Father. We are not born with original sin; we are not responsible for Adam and Eve's sin; only our own, after we are of an age to distinguish good amd evil.

If you have knowledge of nothing you cannot know evil. You can not know right from wrong. You can only have faith that God is right and satan is wrong. If you have the ability to learn. You can learn to deceive. You can learn to play off the good faith of others and trick them into getting what you want. It was not the tree of knowing right from wrong. It was the tree of knowledge. I believe the tree of knowledge is a metaphore for humans taking "satans" advice and receiving knowledge whether it was direct help from an unknown source or what. This is why God didn't want us to have knowledge. We were not ready for it. Therefor knowledge Is a sin. I believe in time knowledge will not be a sin, when Christ returns and good faith is restored among us. There will be knowledge of good and evil but no one will feel the need for evil when good is all around. There will be peace on Earth. We have to learn again to raise up to the next level and be with God. It is your choice.

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.