Just did this today
Flew from SJD to LAX, 3 adults with GE and one infant
Got the paper, went to lane 4
The officer took my GE form and wrote "two" on my old school customs form and off we went...
Btw we had nothing to declare
Basically you need to still fill out the old form as well
Oh, and he was not too pleased lol
But other customs officers said infants should just go through with whoever their ticket is under