What can cause an enlarged clitoris on a child?

Clitoromegaly: The most common pathologic cause of enlargement of the clitoris in a girl is excessive testosterone production, usually secondary to congenital adrenal hyperplasia (a defect in creating a hormone that results in high testosterone levels). The clitoris is embryologically the same as the penis and can enlarge due to stimulation even in children.

Ambiguous Genitalia: Another consideration would be ambiguous genitalia which includes the adrenal hyperplasia scenario mentioned above but also includes various intersex entities. Determination of the child's genotype (xx or xy) would be a good first step in the evaluation. See: http://emedicine.Medscape.Com/article/1015520-overview a bit complicated but lots of info.