Biotin is passed through breast milk, even if you're not on supplements, so technically it's not harmful to your baby, but only your doctor will know how much of it you should be passing onto your child.
ETA: But in all honesty, I would avoid using any unnecessary supplements [especially strictly for hair growth] while breast feeding. I'm 6 weeks pregnant and my doctor instructed me not to use anything that I didn't essentially need. She even told me not to use MN.
Edited by MissMesha - Sep 09 2010 at 3:23pm