SSI recipients are highly likely to be fully aware that any other income other than earned income is going to reduce their SSI quite markedly. Indeed, for the most part, there is only the standard $20 General Income Exclusion (GIE) that stands in the way of a complete dollar for dollar reduction of SSI whenever anyone receives unearned income.
The rules are a little different when Social Security looks at child support payments. First, it should be recognized that the payments that an absent parent pays to the parent living with the child are considered to be unearned income for the child beneficiary: the money is for the child and therefore it is hers. However, despite being unearned income, not all of that money is considered to be countable. Social Security will only count 2/3 of the payments when they calculate the SSI payment. Therefore, of a monthly child support payment of $300 only $200 (300 x 2/3) would be counted before the GIE was also utilized.
There is a significant footnote to that regulation as it only carries for as long as the child is under 18, or under 22 and regularly attending school. If the child support payments continue for the now adult child out of school, Social Security will just count the whole lot, apart from that little General Income Exclusion, of course.