Did you just walk your child through after you completed GE kiosk formalities for yourself? If so, then you indeed violated procedures, because your child would never have legally entered the United States (i.e. the child's passport was not scanned, until the officer caught you at the exit).

If you are traveling with somebody (even an infant) who is not eligible to use the kiosks, that person MUST go through regular passport control processing, including filling out a paper customs declaration. YES...even if it is your child and he or she is accompanying you. YES....even if that child is too young to fill out forms or travel without assistance. CBP needs to document the entry of every person into the United States, regardless of age.

Passport control is an individual processing. Customs is one paper form per family if using regular processing, but is individual processing for GE members using the kiosks.

You were in the wrong here. The CBP officers were correct, and you could have lost your GE membership then and there. And, frankly, it could have been a lot worse. In essence, you attempted to smuggle your child (could have been ANY child) into the United States without completing any entry formalities for the minor. There would have been no formal record of your child's return to the United States. I'm not a lawyer, but I'm pretty sure that is a federal offense (violation of immigration regulations; even worse because a minor is involved).

You might be red-flagged in the future.