Personally, I think it's just rude.
If you're at a concert, a play, or any other function where the audience is sitting and looking at a stage, please leave the iPad at home. It is really, really distracting when people stick that huge 10" screen up in the air to take pictures.
You're not going to get reasonable photos from a distance anyway. Your kid is just going to be a 3-pixel smudge on the screen.
Instead of using your $800 iPad as a camera, go pick up a digital camera with optical zoom from Best Buy. With a zoom lens, your pictures are going to be far superior to anything you can get with a smartphone or tablet.