The children's Last Name at Birth should be whatever name they had at birth.
If a child adopted their stepfather's last name and used it for the rest of their life, put that name in the Current Last Name field.
The Current Last Name should be the one they died with, or the one they have now if they are still living. If they used the stepfather's name temporarily but used a different last name later (such as a woman using her husband's last name), the stepfather's last name goes in the Other Last Names field. If a person used multiple "Other Last Names" (such as a woman who married more than once), they all go in the Other Last Names field, separated by commas.
And you should explain it all in the text of the biography.