mount your camera and lens on a tripod. Use a wireless or wired remote shutter release and trigger - while you go off the camera and play with the baby.
Alternatively mount a teddy bear on the lens. Try some low shots shooting upwards. The common mistake is to take too many top-down - as thats how most adults see babies. These angles are too common. The really good shots could require you to become a baby hehe, ie you crawl or even lie down together with the baby to take.
A kit lens 18-55mm kit lens would be nice along with some flash if its really too dark. Simply point the flash head to bounce roof or 45 degrees behind. If ambient light is good, use ambient as well for variety. Watch out for AF-Illuminator. Turn it off to avoid blinding the baby. Also avoid using too strong flash
Angle, lighting, posing and playing are more impt than what lens you use.