How does SSI collect money that was fraudulently taken.?

We currently have 5 kids living in our home that were supposed to be receiving SSI for their deceased father and we were unaware that they were supposed to receive SSI. Their Uncle was receiving it illegally. They used to reside there a few years back and we learned that he was still collecting their SSI for the past 2 years they were not with him. What will SSI do for him to repay the money that he was wrongfully receiving? How will the children regain that money that was never given to them?