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?
