Last year I had a terrible time the two days I had a similar idea....but with a sticker on my back bumper so I took it off before anybody got hurt and all is back to normal again.

Your sign may not be a good idea visible up your back side but posted out the rear door driver side window would certainy be game.

I've even thought of hanging a small erasable type cardboard "Baby on Board" sign via suction cup on that rear side window while I drive, but to take it down again after I park..... To keep the keys off the paint.

Efficient drivers do it better.
1003 miles a tank personal record. 74MPG calculated. HCH1 CVT