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For your lens side, a fast wide prime is worth considering as mentioned all ready, but I think that the very good Nikon 14-24 zoom is an excellent idea for this situation. For over all shots of the parade "carts" as they are called, you should consider mostly available light, but mixing in just the right small amount of fill flash. For tighter shots of the carts or of groups of people, your flash can be even more important. In any case you need to learn how to use it, including an auto mode with adjustment of the flash compensation to tone down the fill flash amount to blend well with ambient. Some sort of flash diffuser will help, and the ones I like are the FlipIt adjustable ones from Demb.
You should likely look at some sort of flash bracket to allow you to shoot in portrait or landscape while still keeping the flash roughly centered above the camera. I happen to use the Really Right Stuff brackets, but there are other less expensive solutions available.
A second body with the latest and greatest low light performance may be useful, but may not be necessary. If you got another body, you could use the 14-24 on your existing camera with fill flash, and have another body with a fast prime for pure available light. But it can be messy to handle two cameras and a flash with reflector in and around crowds, so you may want to consider only using one at a time. Will you be able to stay close to family? If so, perhaps you could have one of them hold or hold and use one camera while you work with the other. Depending on ages, photography can be a great part of a family experience for the kids. If they are too young for using a DSLR, consider other things like point and shoot cameras for them.
I would work toward getting lenses first, and find out how you want to shoot with them, and then in 6 months decide if you want a new body. That also gives a little more time for Nikon to come out with new bodies which may suit you even better that the current selection. If you were a Canon user I could make specific recommendations, but I do not know enough about Nikon to be of help. I happen to like high resolution cameras, but for what you plan to shoot, low light performance is likely the most important factor in choosing a body.
You mentioned you need for some practice, and you have allowed enough time to prepare. You should look for events or scenes that are somewhat like your night carnival parade situation, and that give you similar lighting conditions. Just wandering for evening or night street scenes of people at events or street vendors or street performers or even store fronts will give you similar situations that allow you to understand what you can do hand held, and learn more about using fill flash. You can also learn more about color balance, and try out a gel on your flash to help improve the lighting blend as Chuck suggested. Victoria should provide plenty of things to act as subjects, and if you stick to the right locations where there is activity, you should be fairly safe. The environment and activity around the inner harbor of Victoria comes to mind for starters.
Not sure if you currently shoot using RAW, but post processing of scenes like you are considering can really help compress contrast and adjust for exposure issues and adjust over all color balance. And you will likely want to use some sort of noise reduction to help with the dark areas of your shots. So post processing of RAW is another part of the practice you should work on with the kind of lighting situations you will encounter.