If you have good 50's, I agree about getting a decent flash. You may want to avoid it at first with the little one, but little ones don't stay little long, and when they get mobile you're going to be shifting from propping the little one up and being artsy to chasing after them as they start exploring their world. Chasing means you'll need flash of some sort, and if you can bounce it, it'll make for better shots then dead on with what you have on the camera.
FWIW, my best shots of my son have been with my 50mm f/1.7 manual, my plasticy FA 28-90 and my Sigma 70-300. I sequed from the 50mm to the FA/Sigma as he started getting mobile. Unless you're a MF ninja, as soon as your kid gets to where they're not holding still anymore, MF can be thrown out the window except for the rare cases where they're sleeping or actually sitting still and focusing in on something.
What I'd suggest if you go for a lens is to get a zoom that can cover a wide range of focal lengths. Save the primes for staged portraits. The 18-135mm has been what I'm focusing on for my 'one lens purchase' come tax season for that exact reason. From everything I read it has a LOT of plusses for kid photography - a relatively fast AF, a relatively silent AF (this is key for candids), and it has the range to capture most of what you'd need for someone running around.
Some of my absolute best kid shots of my son have been with the FA28-90 simply because it allows me to go as long as 90mm, and moves the 'heart' of the lens to around 50-55mm giving me better IQ then an 'at its max' kit lens does. The FA28-90 also has a relatively quick focus and is quiet enough to not draw unwanted attention from him.
YMMV of course depending on what kind of shots you want, but personally I'd aim for something versatile over dedicated as far as child shots go. I've been in the situation where I had the MF 50mm on and suddenly he darts off to do something cute, and has moved on before I've had a chance to scramble back to where he is and try to get focus on him again.
Last edited by Sagitta; 09-30-2012 at 11:30 AM.