First, add the new children through "Add Individual", save the ID number that is created.
Second, if a family was already started you can go to that family's page and click "Edit This Family" add the new children's ID numbers and any other relevant information to the page and click save changes. The family will now have the new children. If a family has not been started you can click "Add Family". Add all relevant IDs and information and click save changes.
Individuals have ID numbers at the top of their page.