They may have used your 2019 return to determine the amount of your CTC---you can try updating your information on the portal if they are not sending the CTC for the younger one.
https://www.irs.gov/credits-deductions/advance-child-tax-credit-payments-in-2021
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
