(defendant's posthumously conceived twins, born 18 months after husband's death, not "children" under Social Security Act as the biological offspring of deceased wage earner; under state (FL) law, marriage ends on death of spouse, so conception after spouse's death means that twins do not qualify as "marital children"; Social Security Administration's interpretation of Social Security Act that qualification for benefits turns on whether children could inherit from their father under state intestacy law was proper reading of SSA's text and is entitled to deference).