What should be the average weight of a 6 year old girl?

Weight. Weight varies considerably with height and body type. Are you concerned about being overweight or underweight? It is important not to place undue pressure on a child because of weight etc. At this young age. Discuss this with her doctor at her next check up. Meanwhile be sure she is on a healthy lifestyle and healthy diet and playing for exercise as much as possible.
About 44 lbs... Using the cdc female growth chart guidelines which the most current posted data is from 2000, a 6 year old female can range from 36 lbs (5% percentile) to about 58 lbs (95% percentile) and still be considered normal. If you wish to use 50% percentile to consider average then that would be about 44 lbs, the growth charts can be found with a google search for cdc growth charts.