peanut gallery

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The hindmost or uppermost section of seating in a theater balcony, where the seats are cheapest.
• noun A group of people whose opinions are considered unimportant.

from the GNU version of the Collaborative International Dictionary of English.

• The cheapest seats in a theater, being typically those farthest from the stage, as in the uppermost balcony.
• In a television show for children, the section where visiting children sit as guests during the performance.
• colloq. Any group of children, or the chidren in a group of people.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic Any source of heckling, unwelcome commentary or criticism, especially from a know-it-all or of an inexpert nature.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (figurative) people whose criticisms are regarded as irrelevant or insignificant (resembling uneducated people who throw peanuts on the stage to express displeasure with a performance)
• noun rearmost or uppermost area in the balcony containing the least expensive seats