From the dashboard, go to the attendance tab or roster tab. Click on the child, click profile and then under enrollment change their status to graduated.
From the app, go to the "kids" screen and then tap the child to view their profile. Then tap on the status row to change their status to graduated.
If you expect the child to return at some point in the future, use the suspended status. You can then reactivate them from the Roster under "inactive" when they return.