u keep hearing that direct flash will hurt a baby's eyes.. most parents seem to be perfectly fine with using the flash on their P&S cameras to take photos of their babies. i think that it should do no harm unless you're trying to take a macro shot of the baby's eyelashes at f/16 with the flash 10cm from the eye!
if u can, borrow both the 50mm and the SB900. use the flash for all the 'standard' event-style shots like group photos to get everyone's faces well-lit and sharp. swop for the 50mm for the candle-blowing, close-up of your kid, candid portraits, etc.