Best way to increase iron?

A couple months back my 2 year old had a blood test done and I was told he had low hemoglobin, as we were leaving the office the doctor said she was calling in a prescription, never told me what it was for or why. It ended up being an iron supplement, which gave my toddler horrible diarrhea. I was never given any other option other than medication, nothing else was ever discussed. We’ve since stopped using the iron supplement and are trying to increase iron in other ways. Does anyone have any tips or advice for increasing iron in toddlers? Or has anyone else had a toddler with low iron?