My daughter just started to learn addition and subtraction in school. Like a true nerd, I wrote a Java applet to generate math worksheets, so that I no longer have to write out exercises by hand.
To try it out, go to math worksheet generator.
Click on the top-right +/- buttons to increase and decrease the level of difficulty, click on the ADD/SUBTRACT button to toggle between additions and subtractions, or click anywhere to generate new exercises. Feel free to print the math worksheets for your own use (unfortunately, this may not work for some browsers).