When traveling to the US with a minor, the official websites ask for a consent letter, "preferably notarized". So what does that mean in practice - will I be at substantial risk of child abduction if my signed consent letter is NOT notarized? Or is an informal consent letter typically accepted at immigration?
Notarizing is a relatively cheap and fast process and it confirms that the identity of the person signing it has been verified by a recognized notary.
Your "risk of child abduction" does not seem like a possibility, but if you raise official suspicions for some reason (within your control or not) you could be delayed. Personally, I have never been asked for such a letter in the first place, with a number of opportunities to be asked, but as always YMMV. They are free to make profiling judgments that might not be acceptable in other circumstances.