It seems to me that framing it all as being “on a diet” is just contributing to the toxic diet culture that we have in this country. If you focus on healthy eating habits and intuitive eating, you build lifelong habits. But anytime you are “on a diet” that means that at some point you’re going to fall off or go “off” the diet, and when you do that if you’re prone to weight gain you will definitely gain weight.
It seems like you are putting your children on a restrictive diet because you are worried that they might get fat at some time in the future. It is certainly hard to think about our children getting teased or made fun of, but the way you talk about honestly feels very anti-fat to me.
It’s one thing to want your kids to be healthy, but all of this emphasis on eating right to avoid being fat is not the right way to do it. Instead, emphasize how they feel when they eat certain foods, which foods give them energy and make them feel healthy. Teach them how to listen to their bodies. You don’t want them to get a complex, but you are perpetuating the idea that if they do gain weight, it’s just about the worst thing you can imagine happening to them.