The Social Security Administration's Office of Inspector General (OIG) has recently completed a report on Households with Multiple Children Receiving Supplemental Security Income Payments Because of Mental Impairments. If you're expecting it to be a big expose of rampant fraud or, indeed, of any fraud you're going to be disappointed. It's mostly about efforts the agency has taken and can take to make sure there isn't fraud.
Please don't make the assumption that there's fraud just because a family exists which has multiple children with mental impairments. That's not a justified assumption. To give a couple of examples where this might occur, consider a couple who adopts multiple children whom they know have mental impairments or consider a family where one child is born with a congenital mental impairment who then have another child who happens to have the same congenital mental impairment. I've seen these cases.