I use my
Nikon 35-70 f/2.8 ($500). No AF-S motor however. For something with an AF-S motor in Nikon's lineup for portraits, I'd recommend the
Nikon 24-70 f/2.8, however that costs $1,700. Since children and babies are small, you might try a
105 f/2.8 Micro. That will set you back $760. You may find it's cheaper to find a used D50 or D80 somewhere. The D40's are really made with the assumption that you use the 18-55, 55-200, and maybe 70-300 VR. Are you shooting for pay or just for fun?