I don't have children (yet). I do have a fiancee with a significant and long-standing eating disorder, contributed to by being raised in an environment where 'fat' was feared and degraded.
She has been battling her own brain telling her to fear weight gain, to restrict food, that her body is something that cannot be trusted and must be controlled and that hunger is not a neutral signal but a dangerous trick that is going to ruin her if she listens to it.
Her BMI is in the 'healthy' range, yet she is exhibiting symptoms suggestive of being in a chronic semi-starved state. Her BMI has never dropped low enough that she ever met the criteria for ED services, yet she has done possibly irreversible damage to her bones.
BMI is bullshit as a way of measuring health, all it does is provide a ratio between height and weight. It was designed as a tool to measure averages, and the population it was based on was adult white men...and not many of those.
Children are born naturally intuitive eaters, with body trust, able to respond in a natural way to hunger and fullness cues. Unless there are medical conditions that disrupt those cues (Prada-willis, ADHD/ASC etc...) their bodies will, when provided with a healthy and varied diet that does not restrict any food groups, regulate themselves.
Restriction, whether that's a 'diet' or just banning certain foods, will lead to inevitable craving of those things and then likely lead to a level of bingeing. That's if you're lucky, unlucky people with a genetic predisposition to an ED could end up triggering that by an initial innocent period of restriction which might not seem that bad to other people.
Diet culture, fear of food, mistrust of our bodies...it's so incredibly damaging. Food should be joyful, fun, can fulfil a ton of different requirements in life both to nourish our bodies and in a social capacity,
Bodies naturally have a set point range that is different for everyone, that's true for children and babies too...otherwise all babies would be born the same weight would they not?
Weight /= health
If the diet being provided to a child is varied and in appropriate portions, and they are active and taking part in varied activities then the likelihood is they are healthy even if their body is larger than someone else's body.
If 100 people were given the same diet and had the same amount of exercise they would not end up all the same size, and some people would have a 'normal' BMI but not be healthy.