Thanks for your question.
Unfortunately, we have no specific information about such an eventuality. However, in Genesis 18:25 we are told that the "judge of all the earth" will do what is right.
In 1 Corinthians 7:14, we read: "For the unbelieving husband is made holy because of his wife, and the unbelieving wife is made holy because of her husband. Otherwise your children would be unclean, but as it is, they are holy."
The children of believers are "holy" and when their parents are called away to judgement, they will be cared for, probably by angels. In the Kingdom, it may well be that parents will be able to finish the job of raising their children who will, if they remain obedient, join their parents in immortality at the end of the Millennium.
That is what I expect will happen. However, since the Bible is silent in regard to this specific case, we must move forward in faith that the "judge of all the earth" will indeed do what is right.
I hope that helps,