The children are added as birth children to their birth parents. The couple's line should show a divorce, or the appropriate chronology. It should be clear when the genogram is constructed to which marriage the children are born. A dotted line can be used to outline a household if not obvious.
With foster children you use a dotted line to attach them to the foster parents, is the a specific line that yo use with step - children when your not required to add the past relationships?
The way I understand it is you show the past relationship status(divorce, cohabitation, engaged, etc) and show the children from that relationship. So, you will have the current couple and their children, if any. And you will also have past relationships plus the children born to that relationship. Hope this helps.