There should be one that simply says "childfree and loving it."
Someone on here had a very funny picture that could be turned into a bumper sticker. It was a pic of a very grumpy, crying toddler with the words "none for me, thanks" alluding to being childfree.
Have you ever seen those "baby on board" stickers? How about a play on it? Here are some ideas. "No baby on board." "No sprogs on board." "No baby on board -- I have a life." "Childfree on board."
Also, one could simply have a no-smoking-esque sign, but with a baby instead of a cigarette.
Maybe one that says "I'm allergic to children."