It's my understanding that we get our hair from our mother's side (maybe that's a myth, but it's what I've always been told.... does anybody know for sure???). What type of hair do you have? Did you have a lot of hair at that age?
I know it's different with all children. My three daughters all have had very different hair growth. My 5 year old has super thick, super long straight hair, that didn't really start growing until she was over three.... now it grows like crazy (she could actually sit on it before I cut it last!) My 3 year old has thin, extremely curly hair, that barely reaches past her shoulders.... and her hair really started growing right around her first birthday. My 10 month old has more hair than either of the other two had at that age (thicker), but it's not growing in length as fast.
Anyway, keep her on a healthy diet, keep giving her the iron supplements and give it some time. It will be harder for you
to notice much change in her hair, because you see her every single day. It might help you if you take some pictures of her hair every couple of months and compare them. That way you can keep track of how her hair is growing.
I hope this can at least help ease your mind a bit....