Generally, Social Security will report the benefits in a separate form for each person. You can see who the form belongs to by looking at the form SSA-1099 box 2, this box will show the social security number of the person that the form belongs to. If the form reports both of your benefits, then you will need to separate what belongs to each person. You will not report your child's Social Security benefits on your return. In TurboTax you will only enter the amount that is part of your back pay.
Retro payments are taxable and reported on the year they are received. You have the choice to determine the taxable amount but choosing to use your current year income or using your income for an earlier year.
The taxability of Social Security benefits depend on if the person received other income in addition to the Social Security. If the only income received is Social Security then it will not be taxable.
