A few thoughts from me:
My skinny brother used to eat AT LEAST 14, yes 14 Weetabix A DAY, every day, caked in half an inch of sugar and full fat milk. He is, and always was, a bag of bones. We had full fat milk, proper butter, puddings with custard or a milk pudding every day, but like others, were very active. We walked/biked to school (a mile away), we spent the whole of the school holidays outdoors, biking miles to other villages, meeting other children to play with, and being out for 8-10 hours a day in the summer. We swam daily in our primary school swimming pool when the weather was warm enough, again spending hours each day down there. I even remember cycling to my grandparents regularly when aged only 5 or 6 and that was a full 5 miles away, up and down hills. Children these days are more interested in playing on their DSs, computers, mobile phones, watching the tv or playing on games consoles. Cycling isn't as safe as it was back then, there are way too many cars to make it very safe in most areas, and children just aren't allowed out to play on their own anymore. Nowadays children expect to be driven everywhere, or at least jump on a bus. Walking is a major deal to them. Things have changed so much.
All food we ate though was home cooked from scratch. We mostly ate meat (fish on Fridays) and veg, including potatoes and as I said, always a pudding. Roast on Sundays, cold leftover meat and veg on Mondays!
I am a childminder and have over 20 years experience. All the children I look after bring their own packed lunches and some bring an evening meal for me to warm up for them. Once upon a time I used to cook for them, but it got a joke when I had half a dozen children from four or five different families, all having different tastes, and me struggling to cook any meal that all the children would eat. I ended up wasting so much. The one and only thing children these days seem to want to eat is PASTA. Children are so lazy with food. They don't seem to be able to use a knife, they all chase food around a plate with a fork. I remember eating the most ropey bits of meat when I was young, but you were made to chew and chew and you were never allowed to spit it out. Meat is a hundred times better quality these days, but children now act as if you are trying to poison them with the smallest piece of roast meat, they just won't eat it. Vegetables too, many will only eat one or two vegetables. Ask what they would like, and they all say Pasta. It's easy. It's soft. No effort to chew or swallow it. But most put either bolognese, pesto or cheese sauce on it, and many invariably smother it with cheese too. So many children end up with constipation too, as they don't have enough roughage in their diets.
Lunchboxes. Well in our day we had cooked school dinners. Pretty good quality too. Again, meat, veg, potatoes and pudding. Very occasionally we had spaghetti bolognese (I think it had just been invented in the 70's 
) but mostly it was potatoes for carbs. And water for a drink.
Childrens lunchboxes today - well I would say about 75% of the children I look after have pretty dire lunchboxes, despite all the media hype about them.
I do have one family whose lunchboxes are wonderful, yesterday they had crackers, cheese, ham, cucumber, celery, crisps, yoghurt & apple. But this is an exceptional luncbox. Many have the chocolate spread, cakes, biscuits, crisps, just a token 6 or 7 grapes if they are lucky.
There are all these awful snacky things on the market too aimed at children, some of which are just junk, some encourage awful table manners. Cheese strings, what are they all about? They encourage children to pick the cheese to pieces. Now they make cheese strings spaghetti too. Ok, I appreciate children need some fat and the protein & calcium cheese offers, but just a slice or two in a sandwich would suffice. Dairylea dips. All sorts of salt rich biscuity/crisp type things to dip into the cheese. Mini packs of chocolate rich biscuits. Individual wrapped cakes. What's wrong with a meat egg or cheese sandwich, a yoghurt and and apple/banana/satsuma or two?
And snacking. What's that all about??? In the good old days you had 3 square meals a day, and if you were lucky, a cup of warm milk (and sometimes a biscuit) at bedtime. You were properly hungry and tucked in heartily to your meal, even the bits you weren't too fond of. These days children seem to want to graze all day long, and it seems to be encouraged. Playgroups, nurseries and infant classes all seem to provide snacks for mid morning and mid afternoon. If the children ate a decent breakfast they shouldn't need to snack. We didn't. I don't think children really understand what hunger really feels like anymore, they are always half full. They nibble on the junk then can't eat the healthy bits off their plates. I only ever provide a bit of a snack after school for those not getting an evening meal until probably 6ish, then I offer fruit/veg cut up, and sometimes some cubes of cheese. It's not meant to be a meal.
Oh dear, look what you've started now!
