To sign sunset, take your non-dominant hand and hold it down flat in front of you, so that your non-dominant forearm represents the horizon. Then take your dominant forearm, hold it up perpendicularly in front of the other arm, and form the ASL letter F sign with your dominant hand. Then slowly lower your 'F' hand by pivoting from the elbow, representing the sun slowly setting on the horizon, with your splayed fingers standing for the last blazing sunbeams right before the sun sets.
Teach your toddler the sunset sign when they start asking you about how or why day shifts into night, and vice versa. If you have the chance to take a lovely stroll by the beach with the family at dusk, teaching your toddler the sunset sign while actually watching the setting sun would be a most memorable experience.
Teach this sign along with the other flash cards pertaining to nature and times of day.