I have a Nikon D5000 I was wondering what is a good lens for taking baby and children portraits
Depending on your budget, any 18-55mm f2.8 would be a good all in one lens. You will be able to get fairly wide shots capturing Mother, Baby, children and surrounding at the 18mm end whereas and at the 55mm end, you will be able to take some really nice close ups of the baby.
The f2.8 (specially if you can stretch to a lens that is f2.8 through the lens range), will ensure that you can take photos of your baby with minimal use of the flash. Set your camera at a higher ISO, shutter at f2.8 and you will be very pleased with your results of how beautiful your baby looks. At 2.8, your pics will also have a creamy out of focus back ground, all adding to the beauty of the photos.
Choice 1 - Expensive - Nikon 17-55mm f/2.8 DX
Choice 2 - Cheaper - Nikon 18-55mm f3.5-5.6 G AF-S DX ED MKII Lens
A 28mm/50mm Prime lens with f1.8 will also be my choice at times, but without the flexibility of telephoto lens, I will be restricted to the surroundings and how far back I can go to ensure I capture the whole scene.
I tend not to use flash with babies. Hope this helps