You do not select the Child Tax credit (CTC). If you are entitled to it, TurboTax (TT) will automatically give it to you. So, why do you think you need to file an amended return to claim the CTC?
Retroactive claims are no longer allowed for earned income credit (EIC), child tax credit (CTC) or American Opportunity Tax Credit (AOC) for any taxable year for which the taxpayer has a taxpayer identification number that has been issued after the due date for filing the return for such taxable year. That is, a taxpayer who files a return with an ITIN and later receives a SSN may not amend prior year return to claim EIC or amend a prior year return to claim CTC or AOC for a dependent who later received an ITIN or SSN.
There are 6 possible reasons; you aren’t getting the Child Tax credit (CTC):
1. You’ve entered something wrong. In the personal Info section, for the dependent, you must select answers that indicate that he/she is your dependent child. If the child was born during the year, say he/she lived with you all year (note: TurboTax changed how this section is done two years ago, you may need to go thru the interview again or even delete your dependent and start over). If the child was born in 2016, you have to answer that he lived with you all year.
2. Your child may be too old (over 16). This comes as a big surprise to many parents the year their child turns 17. A child over age 16 no longer qualifies for the Child Tax credit (CTC). Although a child can still be a student dependent through age 23, and a qualifying child for EIC, the Child Tax Credit expires the year they turn 17 and you no longer get the $1000 CTC.
3. Your income is too high. The Child Tax Credit (CTC) is phased out at higher incomes starting at $110,000 for joint filers ($75K single). You lose $50 for each $1000 (rounding up) your income is over that threshold.
4. Your income is too low. The child tax credit (CTC) is also limited to your tax liability. The CTC is a non-refundable credit and can only reduce your income tax to 0, It can not help you beyond eliminating your tax liability. But, if you have more than $3000 of earned income, some or all of it is usually given back to you thru the "Additional Child tax credit". That is, part of the CTC may be on line 43 of form 1040A instead of line 35 (lines 67 & 52 of form 1040). In the on-line version of TT, the main Form 1040, 1040A or 1040EZ can be seen through Tools, View Tax Summary, Preview My 1040 or go to Print Center and then choose to Preview. The ACTC is calculated on form 8812; that form is not viewable in the online version of TT until you have paid for your return. But, the ACTC is basically 15% of your earned income over $3000.
5. You are the custodial parent and the
non-custodial parent is claiming the dependent this year. The CTC goes with
the dependency, even though the custodial parent still gets the Earned Income Credit, Dependent care credit and
Head of Household.
6. Another possibility is that part of your tax due is not regular income tax, but is self-employment, early distribution penalty or another type of additional tax, for which the CTC cannot be used.
To get a 'second opinion' on-line direct from IRS, try https://www.irs.gov/uac/is-my-child-a-qualifying-child-for-the-child-tax-credit