Our conscience is our realization that what we might do or have done is good or not, but it is not the actual doing or the choosing. On a technical note, for Aquinas conscience is the act of understanding what is right and wrong, though the name may be applied by extension to a habit or power of performing this act of understanding. The virtue of making correct judgments about right or wrong, i.e. appropriate exercises of conscience is called prudence. The reason that this is important is that one cannot do the right thing if one does not know what the right thing is. So, if someone has problems with their conscience, it does not seem appropriate to blame them.
Children do not have fully formed consciences, and do not always understand what the right thing to do is. If a child does wrong because he or she didnt know any better, or because he or she thought it was the right thing to do, we do not (or should not) blame and punish him or her. Aquinas therefore believes that not only is one excused from wrongdoing if ones conscience is in error, one also is bound to do the wrong thing if ones conscience tells one that it is the RIGHT thing to do. He also believes that one has a duty to have a well-formed conscience, one that knows what the right thing to do is. Even though an earring conscious excuses one from doing wrong, one may have done wrong in letting ones conscience fall into error.