I think your question might be more relevant to cultural connotations rather than empirical facts. In any case, post WW2 was not the first time that Japan had to essentally "start over". Recall that the Meiji Restoration was a conscious effort by the ruling elite to reorganize the entire cultural identity of the populace, following the realization that they could not possibly withstand an European invasion. This was around the time when India, China and Indonesia were being colonized by Western powers, and the threat of invasion could not be ignored. The Japanese leadership culled the Shogunate which had been the ruling class for centuries (and essentially Japan's entire culture), in favor of embracing Western superiority and her technology. In less than 40 years, they became a global force to be reckoned with while every other Asian country stagnated in rebelling the invaders.

Post WW2, the remnants of Japan's leadership studied the Meiji era and her rise from obscurity and actually proposed to McArthur the idea of having a self defence force instead of her own military (McArthur initially resisted). This was because it would allow Japan to focus its efforts towards rebuilding instead of defending herself from invaders from Russia and China, by again embracing Western values and its technology to become and economic powerhouse. Recall that Japan at this time had basically been reduced to a blank slate, and as an island state with no natural resources they were in dire need of a solution to support its population. This was what they came up with.

Hence, given that the post Occupation "dependency" was a conscious choice by the ruling elite rather than something forced upon them, it's hard to see how the apparent "childness" in the cultural consciousness could have stemmed from the Occupation. More likely perhaps the feeling of helplessness came from the two Lost Decades after the glorious 80's when Japan was fueled by consumption and excess. Looking back from today, it's possible that a Japanese who lived or was born around that time might have misinterpreted those bubble days as a benchmark for what should be considered "normal", and hence the sharp contrast from the prolonged depression of 20 years contributing to the cultural identification with weakness and hence the feeling of being a child.

If I can add an extra question on top of yours. I know that the reason the US involved itself in rebuilding Japan was to have a friendly buffer state between them and the USSR. Since the USSR has collapsed, the relationship between the two countries has pretty much remained the same. Is it safe to say that Japan is essentially a modern-day vassal state to the US? Were there ever any groups in Japan that felt that way, and accordingly objected/rebelled against it?

Allied Occupation of Japan ended more than 60 years ago.

While the official occupation ended after the Treaty of San Francisco, there are still 50,000 troops stationed in Japan, with around 5,000 support forces and 40,000 dependents. The Japanese Self Defense Forces are, including reserves, around 300,000. Further, the US has a forward deployed aircraft carrier stationed in Japan, the only permanently forward deployed carrier in the fleet. While the official occupation ended decades ago, there is still a significant presence of troops in Japan and their national security is still very much dependent on US forces and the nuclear umbrella.