From Sahih Muslim (Hadith 6759):
Abu Huraira reported Allah's Messenger (ﷺ) as saying:No babe is born but upon Fitra. It is his parents who make him a Jew or a Christian or a Polytheist. A person said: Allah's Messenger, what is your opinion if they were to die before that (before reaching the age of adolescence when they can distinguish between right and wrong)? He said: It is Allah alone Who knows what they would be doing
Reference: Muslim 6759