If you go to the National Geographic's Website for kids, you can click on the NG Explorer Magazine link at the top. This will take you to The National Geographic Young Explorer Magazine (geared towards Kindergarten and First Grade) where your students can listen and read a variety of issues that date back to September 2009. They also have a teachers section for each magazine with lesson plans, worksheets, questions, etc. There is even a section for the Whiteboard.
To go to the direct link just click here.