15 minutes after the service begins, we run report M1025 and bring each room the roster from checkin, and have the room leader confirm each child's presence (if a child isn't present but is on the roster, we assume that the parents didn't take them to the room even though they were checked in and we cross them off the roster at the room; our kiosks are not at the rooms' entry points so this can and does happen a fair bit, especially with very young children). Once the kids on the roster are confirmed or removed, that list at the room becomes the emergency roster in case we need to evacuate the building.
"Die before you die. There is no chance after."
-C.S. Lewis, Till We Have Faces.