Assuming that we are referring to a child as a young girl who has not reached puberty yet, it is unlikely that a diagnosis of PCOS is appropriate. In most situations, girls who have not reached menarche yet, would not experience the symptoms typical of PCOS, including irregular periods, abnormal hair growth and elevated androgens.
However, there are circumstances where a young girl would go through puberty much earlier than normal. This is known as precocious puberty, and is usually diagnosed when a child enters puberty before the age of 8 for girls, and the age of 9 for boys.
One study published by Fertility and Sterility in January of 2009, looked at the prevelance of PCOS in young women who had undergone precocious puberty. Researchers found that women with precocious puberty are "prone to developing PCOS" (Francheschi, Guadino, et. Al)
What does this mean? If a young girl begins experiencing puberty at such a young age, she should be evaluated by her physician, regardless of other symptoms. A full hormonal work-up should be performed to determine if there is an outside cause. She may not reach diagnostic criteria of PCOS immediately, but may be at a greater risk of developing it later in adolescence.
Prevalence of polycystic ovary syndrome in young women who had idiopathic central precocious puberty. Roberto Franceschi, Rossella Gaudino, Alma Marcolongo, Maria Chiara Gallo, Luigi Rossi, Franco Antoniazzi, Luciano Tatò. Fertility and Sterility - 09 January 2009. (10.1016/j.fertnstert.2008.11.016) Accessed online on 1/31/10.