A child when born is body, soul, and spirit. At some point because of the sin nature he becomes spiritually dead. Is the point of spiritual death simultaneous with the age of accountability? Can a child be reborn (saved) before he experiences spiritual death?
Paul addressed your question in Romans 7:9 which says,
“Once I was alive apart from law; but when the commandment came, sin sprang to life and I died.”
In other words, before the age of accountability children are not responsible for their sins, so they automatically have eternal life. They don’t need to be born again because they haven’t spiritually died, and they can’t be born again because they don’t understand what that means. Once they’re intellectually mature enough to understand the consequences of sin, the commandments apply to them. At that point, they die spiritually and have to be born again to be saved.