Having studied nutrition quite a bit for my pharmacy degree there are a few tips I can give that may help. First and foremost, they're teenagers, their hormones are all over the place and that will affect appetite a lot.
Secondly, food in a America is waaaaay more sugar heavy than other parts of the world, this can become addictive, just think about yogurt: plain greek yogurt and regular fruit flavored yogurt have similar amounts of sugar, but the sugar in plain greek yogurt is naturally occurring with higher amounts of protein while the regular flavored yogurt has sugar added in and doesn't satisfy as long (an example to counter this is to buy plain greek yogurt and add half a banana and a teaspoon of honey sweetened jam to the bowl, better flavor, higher fiber content, and should keep a person full until lunch, you can also add a handful of almonds for crunch).
Thirdly, water, oh my gosh water, most people don't drink enough and often confuse thirst for hunger. Have them start drinking a full glass of water before they eat a meal, make it a habit, ingrain it in them, it will help a lot.
Finally, exercise, if they truly have an eating disorder pushing them to eat less may only make it worse (think hiding food in their bedrooms and eating worse things out of sight, you don't want to shame them). If you can't get their intake of calories less by replacing their main source of food with fruits and veggies put them in sports to turn the excess into lean muscle. Soccer, track, dance, swim, anything that has them sweating for an hour several days a week.
Remember it's not about putting them on a diet, diets are temporary, it's about a lifestyle change.