Why do babies of Indian women have black dots on their forehead?

They don't. The 'Red dot' on the forehead is not always only red and nor is it always a dot. The dot is called 'Kumkum' or 'Bindi', and when worn by men it is called 'Tilak' (mark). Usually Hindu women, priests, monks and worshippers wear it. They can take it off and on.