Just a quick question I should have asked... what lens are you using? Keep in mind that f2.8 lenses will work much better in low light and therefore allow a faster shutter speed at lower ISO's... I'm a bit of a lens snob and always have my 17-55 f2.8 on the lens for general shots.. even family photos. Sure.. it's a $1200 lens but my shots are generally sharp and well focused.

Note: if you are taking shots of the kids running around a lot, you should switch to Shutter Priority mode and set a high shutter speed.. adjust ISO as needed to get a decent aperture setting, and don't forget to check your auto-focus mode. If you are still getting blurry photos, you might think about a monopod which will help you stabalize things more.