I am a mother of a 5 year old and have been in a serious relationship for 2.5 years now. You would be surprised at how easy it is to talk to your child. Our daughter refers to me as Mom and as my partner at Vicki mommy. I had her when I was married and thus has a father somewhere. We explain that families are made up of lots of different people.
Some people only have one mom and one dad.
Some have a two moms like us. Some live with there grandparents. This she seems to understand quite readily. It may sound corny but Barney has a video about families. Its a good one because it says the same thing I just did.