One could equally wonder whether the US would have been even more effective during and after WWII if it hadn't squandered the formative years of many of its children on nonsense like shucking oysters when they could have been learning.
Japan’s poor production numbers were not due to lack of successful “pivot” of their workforce, nor of idle hands, nor of lack of skill. Everything was on all cylinders for the war effort.
It was a tiny, resource-poor country of craftsmen and corrupt sycophants fighting an industrial war of attrition, already many years deep in their own theater by the time the US entered. They were doomed even before Pearl Harbor and doubly so afterwards for dozens of reasons aside from their use of child labor prior to the war.
They were towing (handmade, not assembly line) Mitsubishi Zeros out of the jungle by oxen.
The child labor theory is beyond silly.