you must file the amendment within three years of the date you filed the 2016 tax return to maintain your eligibility to amend, so for most people that is April 15, 2020.
the Turbo Tax guarantee only covers the interest and penalties if there is a software 'bug'. if there is an input error by the user, the guarantee does not cover that.
What are the circumstances of your child (age, who did they live with,etc.) that makes you believe you are eligible for the tax credit? is it possible that your answers mean that the child is not eligible? just a thought.
suggesting running your scenario through the IRS website tool to confirm that you are certainly due the credit as that will define your 'next steps'.
https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-f...
