There are two types of benefits to consider in answering this question. SSI (Supplemental Security Income) is designed to provide a safety net for those who cannot provide for themselves. SSDI (Social Security Disability Insurance) is a part of what you get for paying taxes. The latter is earned, the former is a welfare system.
SSDI benefits can be garnished to pay for child support, as they do constitute money you earned. SSI benefits cannot. These are designed to help those who do not have a way of supporting themselves. These benefits fall in a similar category to food stamps and other government programs.