It was possible to get a Birthday pin at City Hall until a few years ago, then I think it was 2006 when they introduced a "Touche de Magie" which I believe left it to the CM's discretion at your hotel, you might get a pin or balloon, etc. However, I have never seen that advertised other than on a CM's blog although I do recall another CM confirming it on this forum.
I would say the best thing is to mention it at the hotel, they were extremely helpful when my DS celebrated his birthday there