There's people upthread chanting "calories out, calories in" - hallehlujah they've solved it!!
It is simply calories in, calories out. That is it. It is possible to remain slim living on takeaways & ready meals & gain weight on a vegetarian diet. Obviously it is not healthy to eat poorly, even if you are slim, especially if you are not exercising. Even salad is fattening if you can chomp your way through enough of it - some of the largest animals on the planet eat no meat or fat at all.
It is simply not true that all teenagers make unhealthy choices once they get the chance, but I can see how they might if they're been raised on the beige crap "children's food" type meals, or given a McD's as a "treat" I'd rather chew off my own arm than eat it. There is absolutely no reason why children can't & don't enjoy proper healthy food, and occasional unhealthy treats - nothing wrong with treats in moderation.
And yes, moderation means once or twice a week, not once or twice a day!
Also a treat doesn't have to be edible - aaargh!
There is a lot of clap trap talked about fruit & veg containing "empty calories" - what a load of bollocks - I worked with an enormous woman on a diet which allowed her to eat literally a mixing bowl full of salad every lunchtime - with a greasy dressing, naturally. She wasn't getting any slimmer, but felt she was making an effort because she would have preferred to be eating stodge.