To use the child check in functionality, there must be something to check into. You can check people into Groups, Events or Services. The Groups area in Roll Call is where you would set up your Sunday school classes, VBS, membership classes etc. The Services area is where you set up your worship services. Each group that you will be checking into must have at least 1 session. A session is the day and time that it meets. For example, if you had a first grade Sunday school class that meets at 9:30 and 11:00, those times would be the session.
To add a group through the check in setup area, click on the plus sign in the lower left corner. You are now at the window to add a group. For more details on creating groups, see Creating Groups.