Hi mate, yeah it's true that in Islam, 'shirk' which is the associating of partners with God, is something that God will never forgive.
"Surely God will not forgive the association of partners (Shirk) with Him, but He forgives (sins) less than that of whomever He wishes." -Quran
Now firstly, a person will not be held accountable for anything they did as a child. Secondly, it's only unforgivable if you die in that state. Therefore, every person has the choice and freedom to believe or disbelieve before they die.
Even though obvious shirk would be actually worshipping let's say another human being, or a prophet (as in Jesus Christ pbuh) or worshipping man made objects, there are also other forms of shirk. There are some muslims who put the prophet Muhammed pbuh on the same level as God, this again is shirk.
"They take their priests and their rabbis to be their lords in derogation of God, and (they take as their Lord) Christ the son of Mary; yet they were commanded to worship but One God: there is no god but He. Praise and glory to Him: (Far is He) from having the partners they associate (with Him)." -Quran
When this verse was revealed a christian man came to the prophet pbuh and said, 'but we do not take our priests as gods'. The prophet pbuh replied did they not make haraam (forbid) what God had made halaal and did they not allow what God had forbidden. He replied 'yes'. The prophet pbuh said did the people accept it? He again replied 'yes'. The prophet pbuh said this is 'shirk'.
This highlights how widespread shirk is today, because Gods laws have been replaced by man made laws and most people including muslims have accepted it. If it was shirk then, it is shirk now.