To log a sleep session in the past, find "Start time" and tap "Set time” at the top-right of the "Add sleep" screen. A time-picker will pop up, and you can select the sleep start-time.
If the sleep session has already ended, set the end time by finding "End time" and tapping "Set time.”
If your child is still sleeping, continue the sleep session by tapping the big circle button that will now say “Resume.”