We have a four year old daughter (myself, my girlfriend and her husband) and we are open with affection about her. We haven't encouraged her to call me Mommy; though she goes through phases of calling me that on her own. We haven't sat down and explained the links. She and I get on wonderfully and she is like my little best friend. I've been in her life since she was two, so she is very used to having me around, very used to my GF sleeping in my bed sometimes, etc.
We are just now starting to deal with things like other parents. I personally do not care what other people think and feel that children will always find something to bully other children about. However, my GF isn't comfortable being open about our V to our daughter's school connections. I respect that entirely, so we leave it at that.
I agree with Marcus that it can be best to let children ask questions when they are ready. I remember going on holidays with my Dad when I was a child and his "friend" would sleep in his bed. He never admitted she was his girlfriend, even though we said we were completely fine about it. Kids pick up on things and I do think they are more concerned about how something effects them; how nice the adults are being, etc.
However, we don't introduce secondary partners to our daughter. We don't want her to become attached and upset. We would have to know a secondary for at least 6-12 months to go there.
me: open poly (31, female)
GF: (41, female) my long-distance, long-term partner
Earth: (35, female) newly dating
Hubby: (38, male) GF's husband
Garcon: (28, male) GF's boyfriend/submissive
“Peace comes from within. Do not seek it without." ~ Buddha