As I said before, if the Adoption Subsidy is from a government agency, it is not taxable and does NOT affect anything on the tax return, either your tax return or the girls.
If the girls' income is only the Social Security (and the benefit from the non-taxable Adoption Subsidy), they are NOT required to file a tax return.
As for whether or not receiving the Adoption Subsidy will affect the amount of Social Security that the girls receive, it is very unlikely it will affect it. However, that is a question that you would need to ask the Social Security Administration.
