I know of 2 . . .
First one is Yoga Child on South Street. I took Mommy and Me Yoga there after my first and really liked it.
http://www.yogachild.net/
The second is a new place I just heard about called Mama Wellness Joint.
http://www.mamaswellnessjoint.com/
Both have pre and post-natal yoga, as well as child prep classes.