We received a Proposed amount due notice from IRS for our 2014 tax return. In that notice, they are correcting our child tax credit, line 52, from $2,400 to $1,150. Why?

The "Why" would be explained in the notice you received from the IRS.  What did the IRS change on the 2014 return that was filed?

Your income is too high. The Child Tax Credit (CTC) is phased out at higher incomes starting at $110,000 for joint filers  ($75K single). You lose $50 for each $1000 (rounding up) your income is over that threshold.

The question is: did you claim the wrong amount, of CTC, to start with or did the IRS change some other item on your return, which raised your adjusted gross income (AGI) and resulted in a  lower CTC, as well as the other adjustment to your return. 

The IRS letter should explain the details.  If you need help from TurboTax support, see https://support.turbotax.intuit.com/irs-notice/audit-support/ 

I agree  .... if you got a CP2000 letter for 2014  then the IRS changed your income which in turn change the credits.  First address the income issue then the credit fallout will solve itself.