So we did it. Here is my 2 1/2 year old son in a little Kimono.
The obi is very poor. In order to have it done for the anime con I just grabbed a contrasting color fabric (fleece sadly). Kenji really liked the spinning to put on and take off the obi. As expected he did not wear it a long time, so we had regular clothes under his outfit.
This was at Sakuracon 2010, the kimono is a vintage kimono from http://www.ichiroya.com/ .
For future costuming it is great to have a real kimono that you can see how it is made.
Some additional pics from our Sakuracon trip and stories coming later.