Who determined that SSI is not income to the Father? It would seem to me that pursuant to Chapter 61, Florida statutes, that SSI would definitely be "income." You don't get to avoid paying child support because you get your money from the federal government or anywhere else. Income is income.
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I think in virtually all states, SSI is considered to be welfare that is excludable from income for child support purposes. Notwithstanding that, many states impute a minimum income to parents with support obligations and/or have a minimum child support obligation that is imposed.