The radiation controversy is a leftover from the CRT (Cathode Ray Tube ) days of TV and computers. New display technologies like those used in most flat screen displays (including the iPad) do not shoot beams of electrons at the screen. Instead there is a bright light behind the display, and you change how much light is let through several layers of colored filters. The only significant thing coming out of the iPad's screen is light.
This means that all you really need to worry about is eye strain. Adjust the screen brightness to a comfortable level for that room. A bright screen in a dark room can cause considerable eye strain. It gives me a headache at any rate. That, and limiting the amount time for constant viewing (like watching movies) should keep your toddler's eyes safe.
