Well, I think the underlying theory is the same for minors as it is for adults. For adults, SSI is (generally speaking) for very poor people who are so disabled that are unable to work. For children, I think you apply that to a child's life. Are they unable to function "normally" as a child. The SSA defines "normally" broadly (and they do not use "normally" as a term of art - that's my word), and since the vast majority of our kids are truly able to live normal lives, they are not disabled for purposes of SSI. SSDI is different in that you must have earned enough work credits to be eligible for Social Security, and your eligibility is not tied to income. I don't know if children can qualify for SSDI or not, but if they can, I suspect the standards for disability as defined by the SSA is the same as for SSI.