I think my child/dependant wasn't added this year to my child tax credit

It depends, if you had less earned income and did not use the option to use 2019 earned income then this can affect the amount of your Child Tax Credit.

Please be aware that only up to $1,400 of the credit is refundable and is phased-in based on your earned income.  In most cases, you must have $2,500 or more of earned income to be eligible for any portion of the refundable Child Tax Credit.  You get 15% of the amount earned over $2,500.  

For example, if you have $6,000 earned income: $6,000-$2,500=  $3,500

                                                                              $3,500 x 15% = $525 Child Tax Credit.

Also, the $2,000 per-child credit is reduced once your AGI reaches $200,000 (or $400,000 if you're filing jointly with your spouse).

Please see What is the Child Tax Credit? for additional information.

You also may use your 2019 income for the Child Tax Credit if your 2019 earned income was more than your 2020 earned income:

1. Login to your TurboTax Account 
2. Click on the Search box on the top and type "Child tax credit"
3. Click on “Jump to child tax credit”
4. Answer the questions until you reach the screen "Do you want to use last year’s earned income?"
5. Note your current refund amount using your 2020 earned income at the top of the screen.
6. Click "Yes", enter your "2019 Earned Income" if the field is blank and click "continue".
7. Compare the new refund amount using your 2019 earned income with the previously noted refund amount using your 2020 earned income.
8. If your new refund amount is lower click "back" and then select "No" for the "Do you want to use your 2019 income to calculate this credit instead?" question.

Also, it is important that the dependent is entered correctly in TurboTax. Please double check that the box "SSN not valid for employment" under your child's SSN is not accidently checked.

If your return has been accepted by the IRS, then you have to file an amended return (1040X) to make changes. But you might want to wait until your refund or payment is processed. Please be aware that you can e-file your 2020 amended federal tax return, but most states require amendments to be printed and filed by mail. Please see FAQ How to amend a tax return for instructions.

Please see Where's My Amended Return? to check the status of your amended return.