I guess, firstly I'd say you need to work out how you're going to manage her disappointment - because it's NICE to have a Kitkat after school, but that's the stuff that's going to have to stop, and you're the boss so you can stop it. But she'll be seeing it as a little sign that you love her and are happy to see her - which is lovely! But as I'm sure you know, once food and love are the same thing, it's hard to separate them.
Replacing food treats with other treats might help. Stickers after school, or even (I know this will sound silly) a flower from a garden along the way or something? Something to still demonstrate that you're thinking of her but not a kitkat.
People are going to say no more chips ever, etc, I think, but it doesn't have to be like that - just less of them. Maybe it's chips OR beans, not chips AND beans, and some cut up carrots/celery/cucumber too. And maybe cut up raw veg with the nuggests, and just a little handful of pasta maybe.
But for starters, try to ditch the crisps and chocolate for a bit, and see if it helps. Easier said than done I know! But if she says she's hungry, offer her some veg, or rice cakes or an apple - if she doesn't want those, she's not hungry!
Oh and Peanut Butter on toast is still fatty but more nutritious than DairyLea or butter and will fill her up for longer. Or try jam, or marmite, or a slice of ham even.