formula milk

Hi everyone My lo is 4 yrs old this year. I've stopped giving her formula since last year. Someone told me it is not good. And I should continue giving the formula milk as it is good for calcium and it will help my lo grow fatter. Currently my lo will only drink about 120mls of fresh or full cream milk everyday. I am personally on the thinner side and it does reflect on my lo. What are your opinions?