I'm not too sure if my experience is helpful, but I had this condition as a baby. According to my parents, I wore a brace some of the time until I started walking. I saw the brace about a year ago and it doesn't look too comfortable! But they said it didn't seem to bug me and I was a very content and happy baby.
My ds has been extensively examined and xrayed and he has apparently not inherited the condition. He would have gone nuts with a brace (and certainly with a cast), so thank goodness for that.
We did visit an orthopod once with ds and he got really interested in my case. He said that I should be xrayed and examined and that people who had this condition should be followed through life to make sure that the joint does not degenerate. I do have some troubles with it, but no time for the doctor visits! Soon I'll have to take care of that.
So anyway, whatever treatment you decide on, I would advise you to keep an eye on it throughout your child's life.
Good luck to you and your family. I have heard some third-hand accounts of this surgery and the cast and it sounds pretty tough on everyone.