ALL kids need one of those documents to cross the border, even an infant. And yes, if traveling with a child that is not their own OR IF THE CHILD IS TRAVELING WITH ONLY ONE PARENT, they need to have a notarized letter from the child's patients (signed by BOTH parents) stating that it is ok for the child to travel with them. IF WITH ONLY ONE PARENT THEY NEED A NOTARIZED LETTER FROM THE OTHER PARENT REGARDLESS OF IF THE PARENTS ARE MARRIED OR DIVORCED. Include as much information as possible...the parents signing the letter should state the full names of the people the child is traveling with, the dates or approx. dates and even the locations if possible...also vehicle info. of the travel vehicle. The more info. the better.
Also be prepared that at the border crossing, esp. coming back into the US, they may ask the kids a lot of questions and not allow you to speak. I wasn't prepared for that