You do Not enter a dependent child's Social Security benefits on your tax return.  The Form SSA-1099 has the dependent's Social Security number entered in box 2, not yours.  If the dependent has no other income they do not have to report the benefits received on a tax return.

why then does Turbo tax require me to enter my 16 year olds magi on my joint return?

Where in the program is it asking for a dependent's income?  Health Insurance?  Child's Income (Under Age 24)?  When entering the child as a dependent?

i think it was health insurance but i can't locate the page that asked for the magi