My 17 year old is eligible for the $2000 child tax credit, but my 10 year old is only eligible for $500. Why?

No.  Your 17 year old is NOT eligible for the $2,000 child tax credit.  In order to be eligible for the $2,000 child tax credit, they must be UNDER 17 (meaning 16 years or younger) at the end of the year.  They would be eligible for the $500 Other Dependent Credit based on age. 

Your 10 year old, would be eligible based on age.  Go back and double check that you didn't enter the birthdates backwards and that you answered all of the questions accurately. You can do this by selecting edit next to each of their names. 

You have that backwards -- or you have mixed up their dates of birth somehow.    You do not get the child tax credit for a child older than 16---you can get the $500 credit for other dependents for him.  You can get the $2000 child tax credit for the 10 year old.  Check your entries in MY INFO and make sure you entered your children correctly.