Can I file for an undeclared children after becoming a citizen of the United States at the USCIS for immigration benefit?

Your question requires an attorney consultation. It is not a simple question that can be answered on this type of forum. There are many factors that would need to be considered and evaluated. Since we do not have all the facts, it is impossible to give a completely accurate answers. Why weren't the children declared? If you stated in your petition for permanent residency that you did not have any children, this could be considered a form of immigration fraud. I strongly suggest that you contact an experienced immigration attorney for a face-to-face consultation and give him/her all of the facts surrounding your situation. She would then be in a better position to analyze your case and advise you of your options.

A parent can certainly file for their child. Of course, the government could also choose to question the parent very intensively as to why the child was never listed before. I've seen cases denied where this was a previously undeclared child. You could have a big fight on your hands to get an approval.