I'm unsure what a "proportional punishment" would look like, in context of a cartoon character suffering a cartoon fate in a cartoon factory.

Sure, the author didn't like the kids. They were wholly unsympathetic. Wonka didn't like them, either -- he waited until the very end to show partiality even to Charlie. The audience isn't expected to like them.

Projections, perhaps, but I would say we're looking at archetypes. Each child represented a particular type of moral excess -- gluttony, avarice, selfish intemperance of some kind. I'm saying they got "just desserts" because their fates aligned with those excesses. Augustus just gets immersed in the chocolate he coveted so much. Violet's impatience manifests itself physically as she became what she desired. Mike basically gets to be on TV for good in a miniature size. And Veruca, oh daddy's rich, haughty darling Veruca, is justly judged to be a "bad egg" on the very scales that measured wealth.

And Charlie managed to navigate the moral hazards and prove his mettle and take that hero's journey that purified him of minor faults. The point of the factory tour was not redemption, but a winnowing. The children had been led to the threshing floor and collectively given a final chance at redemption. Four of them individually failed, but Charlie, as an archetype, represents the redemption of all righteous dudes.

But Charlie had a destiny and didn't need to overcome the deadly sins of the other kids. His humble home was filled with righteous family members who encouraged his goodness. They prepared him to face evil influence when he went out into the world. All the other parents just egged on their stupid children's behavior on the tour. Charlie's weaknesses became his strength and salvation. Charlie's grandfather was a kindly mentor who saw he's good and refused to lead him astray. The other four families weren't merely weak, but thoroughly corrupt and blind to their own sins. They were already destined for perdition before they purchased their first chocolate bar.

So I don't know how you perceive proportionality, but I agree with the commenter upthread -- all seemed to get precisely what they deserved.