Yes, it can be an issue but I live in a border city, cross many times a year for day trips, weekend trips and week long trips with my kids alone and have never once been asked to supply a letter. My first husband is deceased and I am remarried so my daughter has a different last name then me and my son. I still have never been asked to provide a letter or anything else. I have been asked a couple of times what relation she is to me and I tell them she is my daughter and that has been the end of it. She went to Mexico with friends a few years ago and I got a notarized letter from my lawyer giving my permission for her to go with them and, again, she was not asked to provide it at the border into Michigan, the airport or Mexico. I took a friend's child for a weekend stay in Michigan this year, again with my kids and myself alone, and I did request a letter from his dad (who is divorced but has custody) and I just gave it to the border guard when I arrived at the booth and he handed it back. This letter was not notarized, just written by the dad with his phone number on it. I wouldn't really worry about getting a notarized letter - a self written letter from your husband with a phone number where he could be reached at the time when you would be crossing the border would probably be fine.