Jared Bauer , Dec 10, 2008; 01:13 p.m.
The two lenses that you have mentioned have very different zoom ranges - 17-70 and the current 70-300. I think that the range of the 17-70 would be good for shooting kids around the house. I use a 24-70mm range lens around my home with the kids and it works great. It would be great for low light shooting to try and grab a constant aperture lens - one that will give you the f/2.8 at all focal lengths. Sigma makes a lot of nice (affordable) options in that arena. They make a 24-70mm f/2.8, a 24-60mm f/2.8, a 18-50mm f/2.8 and all of these offer the constant aperture. Look into these models.