Sun is the mascot for the Sunnydrop energising candy, which when eaten relased caffeine gives you a boost of energy. This is usually given to kids to keep them playing games or to tired staff members.
Moon is the mascot for the Moondrop sleepy-time candy, which when eaten releases melatonin and makes the consumer tired. This is usually given to kids who can't sleep or rulebreakers.
Sun and Moon work at the Superstar Daycare in the Freddy Fazbear's Mega Pizzaplex.