I would actually take a different approach to Gordon, your biggest issue (I suspect) will be having a fast enough shutter to avoid blur from movement of the children.
I would use shutter priority, and Auto ISO - probably letting it go as high as 3200, a slightly noisy picture is better than one with the children blurry.
If the children (or anyone else) is on stage before the actual performance then take a few test shots to judge how fast a shutter you need to get them sharp - I would start around 1/200, but check as you go - if you can drop to 1/150 it will allow a lower ISO (or wider aperture).
When they are doing dance, etc you will need it higher, though a small amount of motion blur on arms or legs can actually work quite well, as it shows they are moving.
Once you have taken the shots, if some appear too noisy you may find for that converting to black and white works well.
Of course, if you find the light is stronger than expected, and you are easily getting decent shutter speeds without higher ISO, then it's time to limit the ISO and switch to aperture priority.