Your best best for a lens on an DX format (APs-C) camera for those distances will be a 24-70mm zoom. A six-footer will pretty much fill a vertical frame at 50mm and 20 feet; you'll want to be able to go considerably wider in order to capture more than one person at a time from your near-shooting distance, and you'll want to be able to get in just a little bit tighter for the little ones. If you need to rent a 24-70mm/2.8, it would be well worth the money to do so. (If that's the Tamron 24-70 VC, so much the better.)

Assuming that the light is low, you'll want to keep your ISO relatively high — as high as you can stand it, really. If 1600 is usable on the D90, then use it. (If I recall correctly, ISO 3200 is not great, but if you are shooting for a newsletter or website, then a cleaned-up and downsized ISO 3200 shot is many, many times better than no shot at all, or blurry ones.)

You'll want to keep your shutter speed at or above 1/250. Higher for dance segments. Now, I say "you'll want", but what you want might not matter very much. If you find you need to be shooting at lower shutter speeds, then you'll need to pick your moments more carefully. Things moving toward you or away from you won't blur nearly as much as things moving across the frame. A lot of emotional moments happen when nobody's moving at all. There are moments in song when notes are being held, so you can get the expression without worrying too much about the lips moving. Dance has moments of stillness; leaps have moments at the top where the dancer is essentially hanging motionless in mid-air, and a little blur of just the hands and feet will, if anything, make the shot more magical. Practice panning before the shoot — shots where one person is relatively still and everything else is rushing past tell the viewer more than a perfectly frozen moment ever can.

You may find that you can't freeze everything, given the constraints. And you may find, in turn, that that actually leads to better pictures. So much so that when (if) they upgrade the lighting, you'll turn down the ISO instead of turning up the shutter speed.