I am in USA since May 2006.
My wife and kid are still in India.
My wife does not work.
Both of them are dependent on me.
I am going to India on 15th March 2007 and I would be back to USA with my wife and kid on 7th April 2007.
My wife and kid are coming for the first time to USA. So, they do not have ITIN or SSN.
They were in India only throughout 2006.
I am on H1-B and they are now coming on H4.
Can I file my tax return for 2006 as married-filing jointly?
If yes, how and when?

You can file jointly with your wife, if she chooses to be treated as a resident alien for all of 2006.

Not sure you can claim the child.

I did not get the meaning of "if she chooses to be treated as a resident alien for all of 2006". She was in India for all of 2006. How can we proceed with this?

Your wife's physical location is irrelevent; you can still file jointly with her regardless of where she is in the world.