My niece has this problem. She’s under 10 but the doc recommended going to a nutritionist since she was putting on weight too fast.
For the main meal, my SIL always makes a soup first to fill up, so that she’ll eat less carbs in the next course.
For a bread meal, she gives plenty veg, yogurt, etc, so that she won’t eat too much bread.
Nosh is only for Shabbos and even so she doesn’t buy the high fat/sugar nosh.
Plenty fruit.
My SIL has made changes to the family diet and tries not to make an issue out of it, but it isn’t easy. I haven’t heard of any OA for kids