The details of the Hereafter are amongst the unknown matters whose knowledge is only confined to Allah the Almighty and those whom Allah has chosen from among His servants, such as the Prophets, revealing to them things that He’d like them convey to their People.
As for the children who die before reaching the age of puberty, they will, insha’ Allah, enter Paradise. To give evidence and proofs for this, we would like to quote what the late Sheikh Sayyed Sabiq, may Allah bless his soul, states in his book Fiqh-us-Sunnah:
“The children of Muslims who die prior to the age of puberty will go to Paradise. Al-Bara’ ibn `Aazib (may Allah be pleased with him) narrates: When Ibrahim, son of the Prophet, died, the Prophet (peace and blessings be upon him) said, “Verily, there is a wet-nurse for him in Paradise.”‏ (Reported by Al-Bukhari)
Anas Ibn Malik (may Allah be pleased with him) quotes the Prophet (peace and blessings be upon him) as saying, “If any Muslim has three children and they die prior to the age of puberty, Allah will cause him to enter Paradise on account of his being patient over them.” (Reported by Al-Bukhari)
It is clear from this hadith that those children who become a reason for others to enter Paradise, would themselves be in a far better position to enter it, because they are the real cause of the mercy of Allah.
As for children of non-Muslims, they are similar to the children of Muslims with respect to entering Paradise. Imam An-Nawawi says: “This is the sound view held by eminent scholars, and it is supported by Allah’s Words, “We will not punish anyone until We have sent a Messenger.” (Al-Isra’: 15) Now if an adult is not punished for the reason that the truth did not reach him, it is far more reasonable to assume that a child will not be punished.”
A report by Ahmad from Khansa’, daughter of Mu`awiyah ibn Sarim, states that he related her aunt’s following account: “I asked, ‘O Messenger of Allah! Who will be in Paradise?” The Prophet (peace and blessings be upon him) said, “Prophets will be in Paradise and martyrs as well.” Al-Hafiz said that this hadith has a sound chain of narrators.‏”
Having stated the above, we’d like to elaborate on the reward parents will get if they lose a child or more as explained by the Prophet (peace and blessings be upon him) in more than one hadith. Al-Bukhari and Muslim reported on the authority of Abu Sa`id al-Khudri: Some women asked the Prophet (peace and blessings be upon him): “Appoint a (separate) day to (admonish) us.” The Prophet (peace and blessings be upon him) told them: “If a (Muslim) woman’s three children die, they will be a protection for her from Hell.” A woman asked him, “And what if two of her children die?” He replied:”And (even) if two of her children die.”
As for your question about whether this applies to one who loses one child or not, the answer is yes. This has been stated in the hadith reported by Abu Musa Al-Ash`ari (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: “If a child passes away, Allah says to His Angels, ‘Did you take the soul of My servant’s child?’ And the Angels would say, ‘Yes’. Thereupon Allah would say to them, ‘Did you take away the apple of his/her heart?’ ‘Yes,’ the angels would reply. Then Allah asks them, ‘And what did my servant say?’ And the Angels would say: ‘He/she has thanked You and committed all his/her affairs to You.’ Then Allah would say: ‘Then build a house in Paradise for My servant and name it the “House of Thankfulness.” (Reported by At-Tirmidhi)