For children and family portraits, my must have lens is a Nikkor 24-70mm f2.8. This is by far the most used. I find the focal range is perfect for documenting families and I love that I don't have to sacrifice color and clarity as with lesser zooms.
My second favorite is the Nikkor 85mm f1.4. I love, love, love the bokeh from this lens. It's most often used when I'm looking to capture individual portraits of children. Even so, sometimes to mix things up I'll use this lens exclusively on a shoot.