I think saying just overweight is harsh - maybe you should be more looking at malnutrition. There are many reasons why someone can be overweight - including just having a body type that doesn't work well with typical BMI measurements. My 9 yr old daughter is overweight, and pretty much always has been. She eats a very healthy, balanced diet, and has a really good relationship with food (no comfort eating). She is also really active. I suspect that she consistently overeats slightly, and this is why her weight has been creeping up - I remember reading somewhere that consistently overeating by 100 calories a day can result in a weight gain of nearly a stone a year. Things escalated last year, because she started a new afterschool club, and they allow unlimited heathy snacks, so it became difficult to track exactly what she had been eating. In her case, its not a problem of quality (she only drinks water, we don't have snacks at home, although do a little baking here and there, and sweets only on Fridays.), but quantity. Anyhow, she went from "chunky" to very definitely overweight, but it was very hard to address, as we didn't have total knowledge of what she was eating. Lockdown worked well for us, as I was able to track her food intake (and tweak it slowly), plus drag her out for daily exercise (no difficulty in getting her outside, but I wanted to maximise our exercise time with running / cycling). Since then, she has put on no weight this year, even though she has grown several cms. However, I don't want her to know that she is overweight, and I don't want to mess with her good relationship with food. She is suspected to have ASD, and doesn't know when she is full, plus loves the sensation of eating (especially crunchy things), so doesn't naturally self-regulate her intake.
None of this is easy though. It's different for all children - some do put on weight very easily, and have no off switch when it comes to eating. However, having a good relationship with food is so important, and finding a balance between providing a good diet, and not constantly saying "no" to a child who is complaining that they are hungry is difficult. Especially when they are just as happy to eat another apple or a stack of carrots! Feeling constantly deprived will not teach self control - I mean, we know this from most people's experiences with dieting. I also have another child who is at the lower end of normal weight. He does regulate his eating - some days he will eat very little, and other days he eats like a horse. If he were my only child, then maybe I would share some of the opinions on here that it is easy to maintain a child's weight, and that it takes a really bad diet for them to put on weight. The irony is that he has a far more limited diet, and does less exercise than my daughter, so she is probably far more healthy than he is, but she would be the one that people would be more likely to judge.