Thank you so much for the input and recommendation.
I consulted an attorney yesterday and I checked out Jim Hacking's youtube as well. He's so cool giving those free consults online. Unfortunately I do not know how to navigate the site yet to be able to join the live show.
As for the attorney, he advised to list the kid's information as that is what the form asks. The issue of not listing the kid's info in I-485 and I-751 will unlikely come up. And if does, I'll just have to explain. He said the worst thing that USCIS can do is to ask for an explanation and amend the previous documents if needed (which is not).
He added that if I still feel worried I can always bring the child's support court case and payment receipts.
I still would want to consult another attorney for a second opinion though. I wish I can to talk to Jim soon.
Anyway, today I was filling up the application online and I'm stuck with uploading the BC of my stepchild which I do not have. I can't even provide the address information. I am only limited to providing the name and DOB.
Will it be okay to place N/A for the address? And about the BC, is this something mandatory? My spouse do not know the whereabouts of his ex-gf and the child. No communication for the last 18 years.