You should have received an advance for both. Eventually, you'll be able to fix this in the portal, but the feature isn't there yet. The IRS says adjustments to dependents will be in the portal later this summer.
You can access the IRS advanced child tax credit portal here:
https://www.irs.gov/credits-deductions/child-tax-credit-update-portal
It would be prudent to get an account set up there now, so you're ready to go when they add the feature you need.
Nick K, EA