Line 28 on Form 1040

How is line 24 on form 1040 calculated?  My wife and I received $2,000 for our 2 kids per letter 6419 (2 letters, $1,000 each).  So I'm assuming the calculation is $4,000 minus the $2,000 from letters 6419.  My question is, where does the $4,000 number come from?