Children are not little adults because they are still growing and developing. This means that their weight, shape and size can change drastically by the time they leave school. Because of this, we measure 'weight for height' as an age-dependent percentile (a way of comparing individual children to a healthy national average). You can check out your child's percentile in the growth chart book you received when your child was born. Otherwise, you can find charts on the internet.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/how-to-talk-to-your-child-about-their-weight