They dream of the things they hear, taste and feel in their mother's womb. Not as we hear, taste and feel of course but as they do. I know babies can hear music and things around them. They also get to know the sound of their parents voices while in the womb. Obviously they don't know who or what mom and dad is but they do remember the sounds of their voices at birth.
Of course they dream just like you and I would.
I believe babies dream of some sort of past life experience.