Before you even start, make sure you have a fully charged battery and possibly consider buying backup battery, Is there plenty of space on your memory card, and your image quality or resolution is set on high, or large fine?
Be an early bird. A good seat or a good position to photograph is a must. Watch for objects that will obstruct your view (poles, teacher's chair, piano, another parent). Remember the best place to be may not be a seat at all.
Bring a tripod or monopod to avoid shake. A play can last a long time. To avoid a shaky hand consider a monopod or tripod. Have neither try to brace yourself against a wall or a seat.
Use low light setting or crank up your ISO. Some cameras have low light setting, no flash or high sensitivity setting for situations like this when you cannot use a flash or a flash will be ineffective. Some cameras allow you to manually increase your ISO. Remember as you increase your ISO, your image will contain more grain or be less clear.
Flash or not to flash? Remember most on board flashes have a range of 10 to 15 feet. Most performances do not allow flash. Some cameras like digital SLRs allow for larger flashes which can increase your flash range to 30 or 60 feet. If you choose to use a flash make sure you allow the flash to fully charge or recycle before you take the next shot.
Avoid Digital Zoom. Many compact digital cameras, cell phones, and video cameras have both optical and digital zoom. As the camera enters the digital zoom range your image quality rapidly deteriorates.
Meter options. Many compact cameras have different meter systems. Often choosing the spot meter can improve your pictures.
White Balance. If your pictures seem off color, you may need to adjust your white balance based on the lighting. You can manually set white balance or try one of the presets like fluorescent or incandescent. Remember to reset your white balance to auto after the play.
Best Shot Selector and Burst Mode Shooting. Best Shot Selector takes multiple photos when you press the shutter and chooses the clearest one. Burst modes or Live Photo shoot multiple photos and you can choose the best one later. Hopefully one of these images will not be blurred. Avoid these modes when using a flash
Know where your subject will enter or where they will be on the stage. Ask your child the night before or attend the dress rehearsal so you will know where your child will be and what will be happening on the stage. This will also be an excellent practice photo shoot. For graduations know what row they will be seated.
Focus--be aware of your point of focus. As you look through the viewfinder or on the LCD screen, the camera will let you know where it is focusing by illuminating boxes or dots. Make sure that the camera is focusing on your subject not the back of the stage or another subject. Consider tracking autofocus. Remember auto focus area allows you to tell the camera where you wish it to focus.