My 3 Year old daughter will not eat and is losing a lot of weight.?

My daughter is so skinny and it is getting even harder to make her eat. She is so picky with everything. She will look at her food before she eats, and if it doesnt look familiar she wont eat it. I want to know if anyone knows of any good childrens vitamins that I can buy her. She doesnt like gummy bears or flinstones vitamins. What kind of nutrients does she need?