I think the answer to your question depends on what you mean by a “completely egalitarian world”. Depending on the answer, I would ask: why does the kid desire to charge people to earn money in a society like that in the first place?

But yes, I think a lot of people would say there should be a cap on how much more wealth someone has compared to the median, for many reasons, such as the amount of political power it would yield him. The thought experiment uses a harmless activity for earning the wealth, but criticism of wealth inequality is often based on what happens _after_ the wealth is earned too. If in this example you will claim the world remains perfectly (socially) egalitarian and the kid will be benevolent, then maybe we can let him keep his wealth in the thought experiment. But that’s not the world we live in, and possibly never will be.