I have 11 yr old twins, I don't limit their snacking or restrict access to junk food - but they eat a very balanced and healthy diet.
They eat their meals well. My son in particular has an internal body clock for mealtimes that you could use to set your watch, it's that regular. They eat a good range of foods, including tons of fruit and veg.
If they are offered a chocolate bar/sweets they won't always want one. They will also sometimes stop eating a chocolate bar halfway through because they've had enough of it. Don't get me wrong, they love chocolate and sweets - but some days they won't eat any/won't want any at all. Their choice entirely.
They have far better self-control over stuffing their faces with junk than I do. I genuinely wish I had such a healthy approach to food.
For us, not limiting their snacks has really worked. The availability means that it's not "a treat" or some exciting forbidden thing. Obviously when they were tiny it was different, but as they've gotten older allowing them to make their own decisions has proven to be the right choice for us. They are both also a very healthy weight - my son is a bit skinny actually, he's very bony!
If I do pull out food at a park, it's probably going to be a bag of crisps. That's because they won't want food every time we go out. That bag of crisps might have been in my bag for a few weeks. Fruit goes mouldy quickly, but having crisps in there means I know I've always got something on hand so I can just grab my bag and go when we want to go out.
They rarely ask for food when we're at the park, but if we're going to be staying for a while and they're hungry, then why wouldn't I give them a snack? Being hungry sucks the fun out of things. It's no different to being thirsty. I always carry a drink too. It's not fun being thirsty either.
I don't need to reserve crisps as a special treat for my kids. If that's what you do, then that's fine but if you see my kids eating crisps at the park, it's because they were hungry and that's what I had in my bag as an emergency ration. And I make no excuses for that.