Hi I have two children who were 17 and 15 on December 31, 2024 but the child tax credit isn’t populating for them. It’s only populating for 1. What am I doing wrong?

You aren't doing anything wrong.  Only the 15 year old is eligible for the Child Tax Credit.  The 17 year old is now eligible for the Credit for Other Dependents ($500).

Here are all the requirements, per What is the Child Tax Credit?

• "Be under age 17 at the end of the tax year
• Is your child, stepchild, foster child, adopted child, sibling, step sibling, half sibling, or a descendant of any of them (for example, a grandchild, niece, or nephew)
• Has their own Social Security number
• Lived with you for more than half the year
• Didn't support themselves (didn't pay more than half their own expenses)
• Is a U.S. citizen, U.S. national, or U.S. resident alien"

For more information, see Who Can I Claim as a Tax Dependent?

You lost the child tax credit for the older one when they turned 17 in 2024.   Instead you get the $500 credit for other dependents for the 17 year old.   You should be getting CTC for the 15 year old.

https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/child-tax-credit/L2lNhfGDl_US_en_US?uid=m68dfq2u

https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/additional-child-tax-credit/L6xFeMFEf_US_en_US?uid=lqnuygah

https://www.irs.gov/help/ita/does-my-childdependent-qualify-for-the-child-tax-credit-or-the-credit-for-other-dependents