I know BMI calculations have their limitations, but do experts generally agree they are not really applicable for children? Or at least small children? Do small children generally have a BMI that would classify them as underweight if they were an adult (i.e. under 18)?

My pediatrician says my son is "technically" in the overweight category, but her concerns are more on the level of "at risk" at this point. His BMI is above the 85% percentile for a 3 year old, yet, if he were an adult, he would be borderline underweight. Then she says she's not too worried cuz he's so tall... but BMI is essentially measuring his weight compared to his height, so why would his height concern her less??

Just kinda curious, I'm reading conflicting things via Google.