I took my daughter on a girls only trip to Vancouver recently. In order to make sure the border crossing went smoothly, I downloaded a copy of a consent letter for my husband to fill out allowing me to bring her across the border. There are numerous examples of these consent letters on the internet. We got this letter notarized. When the US Customs officer asked to see my consent letter at the border, I was glad that I took the extra time and effort to get this done before our trip.
Due to concern about international child abductions, many countries are now very strongly recommending a notarized consent letter if a child is traveling overseas without BOTH parents or even with just ONE parent present. Take time to obtain all the necessary documents for your international trip by reviewing this website: Documents for Travel. This will save you a lot of time and trouble at the border!