Assalaamu alaykum Brother. I was just wondering, if Allaah created us to test us, then why are children not tested? Why does God let them die young and does He not let them live to test them because it does not make any sense? Or is it possible that some humans did not accept the test, like the animals and angels? Is that a possibility, that some humans did not accept the test like normal humans? I understand that children are a test for us, who are older, but why are the children not tested? Why are they not tested and we are? Why did God save them? Why did God not kill all of us when we were young in order to save us from going to hell? This is what a non-Muslim asked me.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

Young children are not assigned with religious duties because they have not reached maturity and are not eligible to observe religious commands or avoid prohibitions. This is a manifestation of the mercy of Allaah, that He does not burden young children with what is beyond their capabilities because of their limited physical or mental abilities or limited perception. The Prophet, sallallaahu ʻalayhi wa sallam, said, “There are three (persons) whose actions are not recorded: a sleeping person till he awakes, an insane person till he regains his reason, and a child till he reaches puberty.” [Abu Daawood, At-Tirmithi and others]

As for your question why Allaah does not test young children who pass away at a young age, while He tests the adult sane ones with trials in this worldly life, you should know that this is determined by the wisdom and mercy of Allaah. Indeed, Allaah does whatever He wishes, and none of the created beings should question His actions. Allaah, The Exalted, says (what means): {He is not questioned about what He does, but they will be questioned.} [Quran 21:23] Verily, Allaah is Just and never wrongs His creation in the slightest. He says (what means): {Indeed, Allaah does not do injustice, (even) as much as an atom's weight...} [Quran 4:40] The young children whom Allaah admits to Paradise, it is by His mercy and grace, and those whom Allaah shall test on the Day of Judgment from the children of the disbelievers, it is by His justice. This is based on one of the scholarly views on the fate of the children of the disbelievers on the Day of Judgment. Please refer to fataawa 84167 and 2740 about the different scholarly views in this regard. The Muslim children who die at a young age shall be admitted to Paradise; please refer to fatwa 91396.

Whoever shall be admitted to Paradise, this will be out of the mercy and grace of Allaah; and whoever shall be thrown into Hellfire, this will be out of His justice and wisdom; Allaah, The Exalted, says (what means): {And your Lord is not ever unjust to (His) servants.} [Quran 41:46]

As for you statement, "Is that a possibility, that some humans did not accept the test like normal humans," if you mean that some people may be incompetent for religious assignments like the rest of normal people, then you should know that the Prophet, sallallaahu ʻalayhi wa sallam, said, “The pen has been lifted from three: the sleeping person until he wakes up, the child until he grows up, and the insane until he comes back to his senses or recovers.” [Abu Daawood, An-Nasaa'i, At-Tirmithi and Ibn Maajah]

It has been reported on the authority of Abu Hurayrah that the Prophet, sallallaahu ʻalayhi wa sallam, said, “Four types of people will be tested on the Day of Judgment each bringing forth his excuse: a man who died during a fatrah (interval between between the time of ‘Eesaa (Jesus ) and the time of Muhammad ), a very old man to whom Islam was introduced in his old age (while he was unable to understand anything), a deaf and dumb man, and an insane man.” [Musnad Ibn Raahawayh - Al-Albaani graded it saheeh (sound)]

As for the rest of people, the basic principle is that they are competent for religious assignments.

Allaah knows best.