Khan Academy: http://www.khanacademy.org/
Arithmetic videos: http://www.khanacademy.org/#arithmetic
Easiest addition video: http://www.khanacademy.org/math/arithmetic/addition-subtract...
Exercises: http://www.khanacademy.org/exercisedashboard
Addition exercises: http://www.khanacademy.org/math/arithmetic/addition-subtract...