Is a 10-year-old girl supposed to weigh 80 pounds?

It is not possible to answer your question without a little more information. There is no specific weight for a 10-year-old girl (or a person of any age) because weight is related to height and body composition rather than age. For more information about weight in relation to height, and to see charts showing the distribution of weights and heights of children at various ages, please see the page link, further down this page, listed under Related Questions. Alternatively, please feel free to ask the question again and include more detail.