Yes, list the child (your child is your husband's stepchild), and then check no, where it asks if the child is applying with you. You should consult with a qualified immigration attorney for guidance and assistance with this process - you can search for an attorney at www.ailalawyer.org. Best of luck to you.
You list everything and everybody you are asked to list on the Form. Why would you not want to list your child from another marriage? That will have no bearing on the adjudication of the I-751, which will strictly be adjudicated on the strength of the documentary evidence you attach to the I-751.
Behar Intl. Counsel 619.234.5962 Kindly be advised that the answer above is only general in nature cannot be construed as legal advice, given that not enough facts are known. It is your responsibility to retain a lawyer to analyze the facts specific to your particular situation in order to give you specific advice. Specific answers will require cognizance of all pertinent facts about your case. Any answers offered on Avvo are of a general nature only, and are not meant to create an attorney-client relationship.
No. Your US citizen child need not be named. The relevant form calls for children to be named here if the dependent child acquired CR status on the same day as your husband or within 90 days thereafter.