To me, the story of Legotown was not one about capitalism. The questions about whether the builders added value to their Lego brick raw materials, and whether the trading game only allowed zero-sum transactions are irrelevant to the larger point of the story.
To me, the story of Legotown is about legitimate and illegitimate authority.
By what right did the original builders of Legotown assert their authority over the Legos? Why were they able to make the rules about who could use the Legos and who couldn't? Because they got their first? Because they built the best, most elaborate structures? Because they were bigger? There were no classroom rules that allowed them to claim ownership of the Legos. Instead, they took control of the Legos by force and simply asserted their right to make the rules about them.
Yes, capitalism requires adding value to raw materials and ability to trade freely, but it also requires the rule of law that is accepted as legitimate by its participants. If there had been rules about claiming ownership of toys in the classroom, then the builders could have followed those rules and then enforced their own rules about things that they owned. But, in the absence of such a governing framework, they cannot simply assert that they own something, and then expect others to accept their authority.
That's also the real point of the trading game. It wasn't to model capitalism. It was to show the children how it felt to be treated in a system ruled by arbitrary, illegitimate power. By what right do the holders of all the green Legos get to make the rules? Why should their power be considered legitimate? The answer is it shouldn't, because tilting the game in favor of the green Lego holders is obviously arbitrary and unjust. And, if it's unfair for the green Lego holders to make the rules in the trading game, how is that different than the original builders making all the rules about who can and cannot participate in Legotown?
In the end, the children earned back the Legos. But, it wasn't because they agreed to a more "Socialist" set of rules imposed by the teachers. Instead, it was because the children came together and agreed to a set of rules that reflected their values of fairness and justice. And, because the rules were the result of their input, the children considered them to be legitimate.