IMHO it's mainly about snacks. Snacks were invented in the 60s\70s by advertising people. ' Milky Way, the snack you can eat between meals without ruining your appetite'. We don't need them.
As a 70s child I had cereal for breakfast, an apple at break time (sometimes) , sandwiches, crisps and fruit for lunch, then tea. I was ravenous before tea - if I hear the theme tune to 'magic roundabout' or 'Roobarb and Custard' and it takes me straight back to rumbling tummy before the 6 o clock news and dinner. We ate a dinner of meat, potatoes and 2/3 veg (you must have veg of different colours, my mum would say), followed by pudding. Possibly some weetabix with warm milk before bed. Sweets at the weekend, as a treat. Occasional biscuits - two at a time, max. And yes, I know that some kids have a blood sugar thing so they need regular top ups, but that is the exception, and can be handled by a quarter of. Adam sandwich, or a banana , not a giant bag of monster munch or a bar of chocolate.
If we stopped buying crisps and chocolate so that they are just not in the house, and helped our kids to understand that it's OK to feel hungry in the run up to a proper meal then it would have a huge impact on obesity and issues with food. Tiny kids do need a slightly different pattern but bigger kids just don't.
And yes, exercise is key. Just not being sedentary will help. But it's not just about weight, it's about wellbeing-getting out and active is good for social skills, combatting isolation, depression, promoting thinking and curiosity, finding out how your body works, watching and being part of the world - ie all the skills we need as grown ups. There is so much focus on food and exercise, but I really think it should be a more rounded 'healthy way of living' and about getting the most out of being who you are and life in general. Also see my ranty-preachy post on the gcse thread about education
. I'm on a roll this morning.