We wanted something different to the Baby on Board signs, so decided to go with a t-shirt. The size is perfect, unobtrusive for the driver and yet big enough to be seen by drivers behind you. The only downside is that the sucker-pad is not as well made as it could be, and I had trouble getting it to stick to the rear window. Managed to do it finally after ample moisture, and hot-breath onto the window at the same time.
Does the job, quite cheap too!