I got part of my stimulus but missing $600 for a child, how do I go about getting that back?

If for some reason you do not receive your stimulus, you can have it added to your refund. The IRS does not begin accepting returns until January 25 so all stimulus should have been received by then.

Please wait until you are certain there is no second payment, then you can follow these steps, if necessary. 

• After you have entered your tax information and are ready for the Federal Review, stimulus will be the first question.
• If you are trying to trigger it: 
1. go to Other Situations
2. click Done. 
3. The next question will ask about payments received.
• The program will determine the missing amount and add it to your refund.

Please see: Is My Second Stimulus Check on the way? and the IRS stimulus website.