It's giving me the $500 "other dependent" credit, like it is for my child who turned 18. But given that they're a minor, shouldn't I get the full $2000 tax credit like I am for the other 2 children?
Yes, your nine-year-old should be eligible for the $2,000 tax credit (assuming all other criteria are met). However, your 17-year-old isn't eligible for the credit. So, you should be receiving two $2,000 Child Tax Credits, and two $500 Credit For Other Dependents.
If your nine-year-old is the child with the $500 credit (not the 17-year-old), revisit the entry for them. Here's how to do this in TurboTax Online: