Little adds to the insult or sarcasm, if you will.

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http://idioms.thefreedictionary.com/goody two-shoesvvvvvvvvv...
The History of Little Goody Two-Shoes is a children's story published by John Newbery in London in 1765. The story popularized the phrase "goody two-shoes", often used to describe an excessively virtuous person, a do-gooder.[1]
https://en.wikipedia.org/wiki/The_History_of_Little_Goody_Tw...

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goody two-shoes
1. adjective (used as a modifier before a noun) Of or having prudish, self-righteous and/or rigidly moral standards. Many have been critical of the agency as being nothing but a goody two-shoes organization more concerned with telling people how to behave than serving their best interests.
2. An exceedingly and/or haughtily prudish, self-righteous, or rigidly moral person; someone who conforms inflexibly to the rules or the law. Mary is such a goody-two shoes, always squealing to the teacher when one of us does something against the rules. Our gang would have control of half the city if that goody two-shoes hadn't somehow gotten himself elected governor.

http://idioms.thefreedictionary.com/goody two-shoes