When I was slimmer as a child/teenager I actually wanted to put on weight. Nothing I tried would put weight on.
I didn't eat a huge amount of rubbish so typical diet as a teenager at home was:
Breakfast: crunchy nut cornflakes, semi skimmed or skimmed milk, mixed fruit, golden syrup (yes I know!).
Snacks: Rich tea or digestive biscuits - normally 2 with a cup of tea mid morning
Lunch: Mighty white sandwich with cheese and salad and margarine
Snacks: 1 large bag BBQ flavour hula hoops, apple and cheese, Creme caramel pot, 1 x Kit Kat, maybe 1 tube Fruit Pastilles
Dinner: Spaghetti Bolognese with garlic bread
Dessert: apple strudel with ice cream
Snack: late evening - cream crackers with cheese and cherry tomatoes
When at school swap the lunch for sandwiches eaten there or canteen food. I think I had sandwiches. After school obviously have usual crisps/chocolate/Ice Pops.
I walked a lot then. Walked the dog round the park but also to school etc. I did get buses too but we walked because you had to pay for the bus, not free, only short hops on the bus were free. We did do exercise at school but I hated it. Stuff like tennis, volleyball, dancing, athletics, rounders, netball, odd bit of swimming. Seemed to walk a lot in school and in my first secondary school had to walk to the upper school which was approx 10-15 minute walk.
At weekends or in school holidays I still swam a lot - mostly for fun, at seaside or in swimming pools. Was taken or went with friends. Also, ice skated once a week at weekends as local rink had a disco on Saturday afternoons. This was on and off, not every year!
Treat food:
Pizza when out (Deep Pan Pizza Company or Pizza Hut), McDs (my fave was cheese quarter pounder, fries and still Fanta). Popcorn when at cinema
When I started my first job at 17 (size 6-8 then) I'd often have:
Breakfast: quarter of baguette with garlic butter
Lunch: Jacket potato with filling (chilli con carne or cheese), or hummus and veggie sandwich but always with some sort of cake after it - artisan lemon cake (no icing)
Dinner: Ratatouille pasta bake, chilli con carne with rice etc, if being taken out to dinner by boyfriends (quite a lot!), full 3 course meal - usually posh Italian with wine/after dinner spirits
Snacks: chocolate bars or crisps on and off whenever I got out of work
Drinks: if out, Vodka and orange juice or vodka lemonade and lime
I recall in my 20s - at weekends I'd have a carton of e.g. New Covent Garden Broccoli and Stilton soup with bread for lunch. I did eat stuff then but was clubbing so lots of cocktails - Sea Breeze (vodka, grapefruit and cranberry juices).
At 18 I started a weekly aerobics class but walked a lot or got bus/train.
20s - worked in London so lots of public transport but walking with it. Did an aqua aerobics class a week plus swimming and also clubbing - which was 1, 2 or 3 times a week so loads of dancing.
I've always been quite active.
I do think (even though it doesn't look it) as a child/teenager I snacked a lot less. I recall coming in from playing out and getting a glass of water or apple. We did have a biscuit tin but were told not to spoil appetites by eating before dinner. we did have Eggy Bread etc but treats.