According to Disabled World, a rough average for what a 7-year-old girl weighs is 49.5 pounds at 47.7 inches tall, and an average 7-year-old boy weighs 50.5 pounds and stands 48 inches tall. However, they note that these are only rough averages because children grow rapidly and at their own individual rates, so there is no one amount that children of any age should weigh.
Because of children's rapidly changing height, weight, bone composition and body fat, rough estimates of body weights should be used in conjunction with body mass index calculations for age and sex. According to the Centers for Disease Control, BMI, which is calculated as weight in pounds divided by height in inches times height in inches, times 703, is a better marker for healthy weight for children and notes that although BMI is not a diagnostic tool, it can be used to screen for overweight and obesity in children. A BMI between 14 and 17.4 falls between the 10th and 85th percentile for a boy who has just turned 7, and a BMI between 13.79 and 17.62 falls between the 10th and 85th percentile for girls who have just turned 7. The BMI percentiles change monthly for children from 2 to 20 years of age.