Supplements for 4years old

Hi mummies, any recommendations for best supplements, vitamins c to boost appetite and immune system for my 4 years old? I have been giving my daughter sambucol and guardian kids vitamin C but its still the same, she always eat less , always want to eat the same things which is her favorite food like fishball soup chicken rice nasi lemak fried beehoon etc. She dont like chicken and don't want to eat chicken and vegetables. But she love fruit alot. And she always sick every month since she started school in March last year and its gonna be 1 year soon of her schooling and she will always be sick everymonth and her cough and flu is still there for the long time didnt recover . Went to PD get antibiotics, cough flu medicine but its the same. Her cough and flu is on and off and its already been month. #firsttimemom #advicepls #pleasehelp