Well, since you're profile says you're from the united states, I have no idea of the weather conditions, so it would really help to know. Also, it would be good to know what the substrate is, as that can have a real effect on how to mount it.
If you want a clean look....... think about at least 3/4" pvc.... if not 1". Make it 4" longer and rout your posts and slide the sign inside of the routed areas and drive 2 or 3 screws through the post, sign and almost out the other side, just not quite. Make a plug for the screws. 1/4" anything will not work. Too long of a span, especially for the little ones.