Scriptures are clear that those dead in Christ will rise first, then those then alive will follow to meet the Lord in the heavens. 1 Thess. 4:16-18. However, the issue of children under the ‘age of accountability’ seems vague at best. It is my witness that the Lord takes these babes with him as well because His love extends to the heavens and his truth to the clouds. Ps 57:10, 108:4. While He is surely just, His judgment is covered by His mercy, thus the ‘Mercy Seat’ rather than the ‘Justice Seat’.
I don’t think taking children under the age of accountability in the rapture is an act of God’s mercy. I think it’s a requirement of His Justice. You and I will go because we’ve accepted the Lord’s death as payment for our sins. Allowing the penalty for our sins to be paid in full by His death is a demonstration of God’s mercy. He didn’t have to do it.
By definition, children under the age of accountability have a clean record. No sins have ever been charged against them, so they have no penalty to be paid. A just God can do nothing else but include those who are righteous on their own account along with we who were made so at the cross.