Good questions.
Hopefully the stepchild realizes you are not his/her bio father? I very much doubt the CO will ask anything related to the bio father (name/DOB etc), since that info would be in the record, and that person has not been part of child's life.
My impression is that interviews for children are nothing like interviews for adults. The kid is only there due to the choices made by a parent. CO may want to know generally how things are between the child and you, and what he/she thinks about moving to the US.