Normal BMI values are for adults. Child BMI differs somewhat sharply depending on age and gender.
This [1] is a tool from the CDC for measuring the BMI of children. For instance a 9 year old with a BMI of 22.5 (in the middle of the normal range for an adult) would be considered obese. A BMI of 17.6 (very underweight for an adult) would mean they're more overweight than 70% of other children their age, but it's still considered a healthy weight - just on the high side. By contrast dropping down just 1 BMI point (to 16.6) puts the child at the 55th percentile, meaning it's a perfectly average weight.
This [1] is a tool from the CDC for measuring the BMI of children. For instance a 9 year old with a BMI of 22.5 (in the middle of the normal range for an adult) would be considered obese. A BMI of 17.6 (very underweight for an adult) would mean they're more overweight than 70% of other children their age, but it's still considered a healthy weight - just on the high side. By contrast dropping down just 1 BMI point (to 16.6) puts the child at the 55th percentile, meaning it's a perfectly average weight.
[1] - https://www.cdc.gov/healthyweight/bmi/calculator.html