As I understand it, altering your withholding will not affect how much hybrid credit you'll get. It's based on your Alternative Minimum Tax determination, even if you do not need to pay AMT. The hybrid credit comes off after all other credits, so the child credit would likely reduce your hybrid credit (the $2k would come off the $3k credit). However, I'd say talk to a friend who's an accountant to make sure you can maximize your credits and deductions.