The child tax credit would not be available if the child was age 17 or older at the end of 2017. If the child was under the age of 17 then you may have mistakenly indicated that either the child did not live in your home for more than 6 months or that the child provided over one-half of their own support.
If the child was age 17 or older then there would be no reason to amend your tax return. Otherwise you will need to amend the tax return to correct any errors for the dependent children. An amended tax return can only be printed and mailed, it cannot be e-filed.
See this TurboTax support FAQ for amending a 2017 tax return originally completed and filed using the TurboTax online editions -
https://ttlc.intuit.com/questions/4252204-how-do-i-amend-my-2017-turbotax-online-returnNote - The 2017 TurboTax amend software does not support Windows Vista Operating System. You would need to either upgrade your OS or use another computer that has Windows 7 or later to amend your 2017 tax return.
