I believe the only way to "relieve" an employee would be to send them home. However, that isn't exactly what you'd want, there really should be a "break" option...
Also, a way you may fix the cleanliness of your store (if you're not already) is by assigning tasks to the employees. You can find them on the interaction menu when you click your employee, once they've arrived for their shift. I believe you can only assign one task per employee, so if you have multiple employees only one can clean at a time. The nice thing, though is that you, as the user, do have the option assist by dragging and dropping the dirty dishes into the trash or sink. Also, your current employees may not be using the trashcan because its full (?), not sure, but I thought I'd throw that out there. 🙂
Personally, I LOVE the fact that sims are now able to start their own businesses, but I still feel there's a little tweaking necessary. I find it strange that once the store closes, the employees leave. They should have to stay after until their work is done 🙂 I hope this helps!
