I'm not sure what the issue is? In general, my view is that:
(a) it's good for children to have their own money (and to be allowed to spend that money), as it teaches them the value of money, how to prioritise spending, how to save and wait for what they really want. For younger children, it's a helpful aid in reinforcing numerical skills, understanding how to round up and work out what change they should get, learning the value of different coins and notes etc. Personally, I found cash was better, as I was really surprised my young child didn't understand the link between cash, which one has to earn, and a credit card. He thought if you swiped a card, you weren't actually paying anything for the item. (I can see why transitioning to a card would be useful for a teen, as card and online payments are, unfortunately, the way much of the world works now, so they need to learn that skill as well.)
(b) a child's money, whether they receive it as pocket money or, as older teens, earn it in part-time jobs, is their own to spend on what they wish.
I understand your concern about sweets but, to me, that's a conversation (or many) about everything in moderation, how to keep healthy and why that's important. It's not about spending money. As for buying a t-shirt...who cares? Why is it bad for a child to spend money on clothes they like? I guess if it was a t-shirt that had an offensive logo or slogan on it, that would warrant some discussion around respect and our wider responsibilities (and, possibly, the law, depending on what was on the shirt) rather than specifically about spending money.