If you have more than one lens and you shoot events or candid shots, I highly recommend getting a second camera. I used to have only one camera and two lenses (Tamron 17-50 VC and Sigma 50-150). I liked the Sigma 50-150, but it was too specialized, so whenever I had the choice between the 17-50 VC or the 50-150, I almost always chose the 17-50, so I got very little use out of the 50-150, even though I liked it. I finally decided to get a second camera, and it made a big difference in my photography. I used my Sigma 50-150 far more often and I got a greater variety of shots.
Buying a second camera isn't just for gearheads. I honestly believe it would be useful for the majority of family photographers. If you shoot events and candids, you probably don't have time to switch lenses (or if you do, you're probably missing some moments). Plus, if you don't have to switch lenses, you will get less dust in your sensor, and it's less likely you'll drop your lens by accident. Moreover, if anything happens to your primary camera, you will still have a backup.
A second camera doesn't have to be expensive either. You can buy a very capable Nikon D70 for $200 or less these days.
If you're looking for a second body, I recommend getting one that has different capabilities from your current camera. Here are some examples for Nikon (Canon has equivalents as well):
- Shallow depth of field and great low-light performance: any full frame DSLR.
- Good low-light performance: Nikon D5100 or Nikon D7000.
- A weaker anti-aliasing filter for greater sharpness: Nikon D800E, Nikon D300, or Nikon D300S
- Unlimited sync speed: Nikon D40, Nikon D50, Nikon D70, Nikon D70s
- Pop-up commander flash (so you can use an external flash wirelessly): Nikon D70, Nikon D70s, Nikon D80, Nikon D90, Nikon D7000, Nikon D300, Nikon D300s, Nikon D700
- Built-in focus motor (so you can use lenses that don't have focus motors): Nikon D70, Nikon D70s, Nikon D80, Nikon D90, Nikon D7000, Nikon D300, Nikon D300s, any full frame DSLR
- Tilt and swivel LCD (so you can get shots from unusual angles): Nikon D5000, Nikon D5100, Nikon D3200 (with wireless adapter)
- High dynamic range: Fuji S5 Pro, Fuji S3.
- Very high resolution: Nikon D3200, Nikon D3x, Nikon D800, D800E
- Compact body: Nikon D40, D40x, D50, D3000, D3100, D3200.
- Video (if your current camera doesn't have video): any recent DSLR.