Is a 5 year old too young to be grounded for a day?

Five years old is not too young to be grounded for the day if the offense happens that day. Don't punish her tomorrow for something that happens today. Tell her that the next time she sneaks chocolate, the rest of it will be thrown away. If she behaves badly, she can be sent to her room for the remainder of the day but punishment should not extend into the next day.