If I understand correctly from your post you are with the childs father and are also "dating" another female?

While I am certainly against exposing a child to a BDSM relationship at a young age, as long as you are not explicit with it and the child doesn't feel it jeopardizes the relationship with the father you should be able to include your girlfriend in your family as a sort of "aunt."

When I was about 14 I had a friend whose mother was divorced and had a room mate. His mother's room mate, they all lived in the same house together, was a female and even to my young eye it was obvious they were gay. They were never open with it around us, and I never mentioned it to him, and he was comfortable with the situation and got along quite well with his mothers' friend. As long as you don't do anything that could traumatize the child they are quite happy with "extended family" situations that adults would be shocked at. Think about "Scooby Doo" two men, two women and a great dane driving around the country in a van? If kids can accept that as normal...