Hello Rinta,
If you search on this site about ideas of birthday celebrations, you will get plenty of ideas. Even similar discussions you will see on the right side of the posts.
Nonetheless, as your staff number is less, you can celebrate by planning innovative and joyful games at each birthday so everyone will be looking forward to this event.
Let your Senior / Employer have an informal one to one meeting on the employee's birthday so his binding with the organization will be strong.