Hi, Lee.
It is highly unlikely that her children received any SSI benefits. Remember, those are funds from the Treasury.
If she was widowed, she could have been receiving survivor benefits from SSA (the Social Security Administration). That provides a stipend for children until the age of (I believe) eighteen, or twenty-something if they are a full-time student.
When the kids fall off of the rolls, the payments cease to the widow.
Not familiar with her situation, but ... if received from the Social Security Agency, that would probably be it.
Perhaps the checks, or correspondence were from "SSA", Social Security Administration.
Don't know, but it's strange if she was receiving SSI.