What should be the average weight of a three and a half year old girl?

The child is three and one half years old, and unless she has been diagnosed by an MD as obese, she should weigh what she weighs.

If you are an adult asking this question about your child or a child over which you have influence in the dietary choices of that child, please be aware that your nutritional responsibility to a growing body far surpasses any vanity about the way the child looks.