We have to be super clear here…a child, born in the US of diplomats with A2 visas, is not considered a citizen of the US under the 14th amendment, said child has to apply for permanent residency (Green Card).
Have you done that? https://www.uscis.gov/i-485
If so, I can’t answer your FAFSA question, so will call @kelsmom and @BelknapPoint