Hi,
Okay, first the good news....
For fireworks and night shots of static objects (things that don't move) you want a tripod, a remote shutter release (so that you can fire the camera without actually touching it), and learn how to use bulb mode on the camera. You don't need a heavy-duty tripod for the D3100 and the kit lens, and remote shutter releases are cheap.
For low-light shots of things that move (like dancers on a stage) you want a faster lens. "Faster" meaning a lens with a wide aperture. And lenses=$$$$.
General low-light tips (for things that move): keep the aperture open wide and crank up the ISO. And use the slowest shutter speed you can get away with,
without causing motion blur--either from the subject or from your hands shaking the camera. You're trying to get as much light as possible to hit the sensor. A high ISO makes the camera sensor more sensitive to light. A wider aperture allows more light to hit the sensor. And a slower shutter speed keeps the sensor exposed to the light longer, allowing more light to hit it.
The
35mm 1.8 Nikon lens is a great deal for the $$. However, it's a prime lens (no zoom at all to it--you have to physically get further from or closer to your subject). It's a great focal length for Disney dark rides on your camera, though. But it might not have enough reach for a dance performance, depending on where you're sitting.
There is also the
50 1.8 G AF-S, which would give you just a little more reach (not a ton) for dance. Still a prime. Not as good a focal length on Disney dark rides, IMHO. Depends what you like to shoot though... hard to get a complete dark ride scene with this one, but does let you get some closer-up shots within each scene. (Note: there is another 50mm 1.8 lens that is cheaper yet, but it won't autofocus on your camera body. For Nikon lenses, if you want the autofocus, look for AF-S in the title of the lens.)
Fast telephoto lenses, especially zoom lenses, are expensive. Especially fast telephotos with a focus motor in the lens (which you need if you want them to autofocus on the D3100. If you're willing to manual focus, you can look at older used lenses without in-lens autofocus).
Now, depending on the lighting on the stage, you might be able to get away with a slower lens if you set your ISO nice and high. I managed to get a few decent shots at the Lion King show in Disney one year with a slow lens and the ISO cranked up on my D60. So if you don't want to shell out money for a faster lens, still give it a try!