We're only children of God if we've accepted Christ as our Lord and Savior.

To say that the age of accountability is unbiblical is to say that God sends infants to hell. There's no evidence that He does. On the contrary,

(2 Samuel 12:21-23 NIV) His servants asked him, "Why are you acting this way? While the child was alive, you fasted and wept, but now that the child is dead, you get up and eat!" {22} He answered, "While the child was still alive, I fasted and wept. I thought, 'Who knows? The LORD may be gracious to me and let the child live.' {23} But now that he is dead, why should I fast? Can I bring him back again? I will go to him, but he will not return to me."