"Social Security makes payment under the SSI program to disabled children under 18 whose families have little income and resources. SSI payments are based on need rather than prior work and may be paid to children regardless whether a parent is retired, disabled or has died. These benefits also continue as long as the child is disabled and has little income or resources.

"A child under 18 is considered disabled if his or her physical or mental condition is so severe that it results in marked and severe functional limitations. The condition must last or be expected to last at least 12 months or be expected to result in the child's death. And of course, the child must not be working at a job that we consider to be substantial work."

From: http://www.ssa.gov/kids/parent6.htm