First, one income is pretty doable for 2 people. With child, I would say 1.5 income would be helpful.
You'll get a subsidy from the government as well that should help a bit.
There is a good chance you will be placed together and a good chance that as long as you pass the application, it won't matter if you have a child.
Here is a fave blog of mine of JET family:http://dustinandlaura.blogspot.jp/
I have a blog as well, but a few months outdated...we had our baby here in Japan if you are interested in the whole process:http://industriousants.wordpress.com/
Just check out the Made In Japan headings.
Cheers and good luck!