I think that perhaps there are two issues here and the OP has raised concerned about both of them.
The first is the speedy consumption of ‘treat’ food and drink beyond what is reasonable and fair. It seems as if the greediest and least scrupulous are getting the most unhealthy diet.
Secondly is the issue of filching and taking the treats of others.
I sympathise with the OP over both as she see the situation getting out of hand.
A solution might be to restrict junk food and treats by stopping buying them and explaining that it is because the rules are being bypassed. Secondly, explain exactly why it is wrong to take the treats of others and by laying down rules for punishments in the future.
You might want to make a point ahead of this by taking a valued possession from the filchers temporarily and then asking the child in question how they felt when their, say phone, went missing. Then point out that is how they are making others feel before returning the item. Connect this with the future regime and say that until their behaviour improves and they can be trusted no treats will enter the house.
It will be hard on the OP because no one wants to deny their children desirable things but they must learn not to be selfish to make their way in life amongst others.
Imagine what would happen if they took other children’s treats at school. If the answer is that they would only steal at home then the situation is that the children are seriously ‘taking the mickey’.
Having said this, most children behave in the same way. I stopped buying chocolate because the children took it. You just need to make a stand and let them know it isn’t acceptable.