If they realize their need of a sinner, they are already accountable.
realizing that they are a sinner is not just head knowledge(or mental knowledge)that is not the requirement for salvation--but it takes that head knowledge getting to the heart and becoming a reality--also need of Jesus is in the same likeness--and to have that they must have proper understanding for God to put that in their hearts through conviction of the Holy Spirit.
Its not a bubblegum chewing event of saying this prayer or doing the things people think you need to do.Its a DEEP CAUSED GRIEF by the Holy Spirit inside of a sinner's heart drawing them to Jesus and revealing to them who they are--who he is-
Now a question for you--if a 4,5,6 year old doesn't even know the concept of what Sin is--Sure they know some things to do that are right and things not to do that are wrong--but that's not the knowledge of good and evil.
Maybe they feel bad about doing something wrong(for various reasons--maybe not wanting to get grounded or spanked?)maybe they are just going down to the alter to satisfy their parents and make them happy?
Then they might get that Candy they otherwise would not get ?
But to be convicted by the Holy Spirit is a different thing indeed:godisgood: