Yes, you can have a breast cake for your 6th birthday
My daughter has been talking for two months now about having a breast shaped cake for her birthday. She is turning six. She is not still breastfeeding and she doesn't have any younger siblings, so I'm not sure what gave her the idea. We decided to do it. My mom thinks it is terribly inappropriate, but I feel like it is her birthday and she should get to have it as much the way she wants it as possible. I think it turned out great.
EDIT: Yes, I did serve it at her party. No one said anything negative about it. One child asked her mom what it was, and her mom told her it was because breast feeding is wonderful. The beginning of August is breast feeding awareness week, so I think that helped. Several friends commented on how cool it was and took pictures.
EDIT AGAIN: I'm not concerned with why she wanted a breast cake. Originally she asked for a "nurser" cake. When she was breastfeeding she used to call them "nursers" so I assumed that it had to do with that. Some of her friends do have younger siblings who nurse and we also have several books about nursing. I just don't know for sure what made her think of it. I asked her two months ago and she first said it. We didn't talk about it at all after that until it was time to make the cake. I wanted to see if she would remember that she wanted it that way. I also figured by this time she might not remember what made her think of it in the first place. I don't find it strange or weird that this would be her request. I think it is empowering to her as a future woman that her breasts have the power to feed and sustain a baby.
ANOTHER EDIT: added another picture for those who thought this was a troll post.