If the dependent child was age 17 or older they are not eligible for the additional $500 payment.
Go to this IRS website for stimulus payment information FAQ's (Question 30) - https://www.irs.gov/coronavirus/economic-impact-payment-information-center
A30. Payment amounts vary based on income, filing status and family size. If you filed a 2019 tax return, the IRS used information from it about you, your spouse, your income, filing status and qualifying children to calculate the amount and issue your Payment. If you haven’t filed your 2019 return or it has not been processed yet, the IRS used the information from your 2018 return to calculate the amount and issue your Payment. The IRS is not able to correct or issue additional payments at this time and will provide further details on IRS.gov.
If you did not receive the full amount to which you believe you are entitled, you will be able to claim the additional amount when you file your 2020 tax return. See the question earlier in this section on determining the amount of your Economic Impact Payment.
Keep the Notice 1444, Your Economic Impact Payment, you will receive regarding your Economic Stimulus Payment with your records. This notice will be mailed to each recipient’s last known address within a few weeks after the Payment is made. When you file your 2020 tax return next year, you can refer to Notice 1444 and claim additional credits on your 2020 tax return if you are eligible for them. The IRS will provide further details on IRS.gov on the action they may need to take.