Let take a child competing with other classmates as example
as an example
I would agree that, if writing an academic paper, it is better to write "Let us" than "Let's".
Many academics won't mind seeing some first person tenses in your paper, but many also do mind. Personally, I avoid first and second person tense when writing academic papers. So, I might just use the gerund form in your examples.
Instead of "Let us consider" - "Considering..."
Instead of "Let us see" - "Seeing..."
That tends to be read in a more universally friendly manner.
the highest - remember the rule of superlative sentence
The reason is that human complete proliferation and preparation to face future challenges could be achieved through those ways
this word or your last sentence makes you unconsciously open a new matter in the discussion, which means you broaden the limitation into human life rather than focus on the children until the end of your essay.
overall, i see a good elaboration on your essay, of course with several errors, but I guess it is still possible to make you to earn a good score.
keep writing mate, 7 more essays are waiting for you :D