My daughter is so thin. She has a low weight improvement. At the age of irhes got 15kilos til now shes turning 7in july she has 16 kilos. What vitamins?

? Failure to thrive. Her weight has dropped from well above the 5oth and close to the 75th percentile (compared to other girls) to below the 3rd percentile. This might be something serious, such as a hormone deficiency. Suggest u diascuss with her doctor. My suggestion would be to see a pediatric endocrinologist.
No specific vitamin. If your daughter is 16 kilos at 7 years of age, she is below the 3rd percentile for wieght. Vitamins are not the answer. They are not going to help her wieght. To better answer your question, her hieght, your height, and dad's height would help. Also, it is imprtant to know how her weight has been tracking over time.
Low weight. Vitamins definitely are of no help. As mentioned above if your family is made of of mostly small, thin people she may be perfectly normal. If she has always been around the bottom of the percentile curve but is following the curve nicely for both height and weight this could be fine for her, unless she has other health issues.