I just did a brightness comparison with the iPad 2 (left) and iPad 1 (right).
The top photo shows full brightness. The iPad 2 is brighter.
The middle photo shows about 80% brightness, where I usually leave it. Here the iPad 2 is still brighter.
The bottom photo shows minimum brightness. Here the iPad 2 is clearly dimmer.
So I think the iPad 2 has greater "brightness range." Not sure about the iPad 3, iPad 4 or Mini though. If that also holds true for the newer iPads that may be enough for her.