Nobody said anything about "a good idea" in this. But logically it is true that your child has no possibility of going to hell if s/he dies very young, but does have that possibility if you let him/her grow up-- that is, IF a very young child cannot be guilty of sin. Do you logically disagree?
Yet couple that tenet with "once saved/always saved" and you cannot go to hell for committing murder if you kill your child. If both those beliefs are correct... that's the end of any debate about the reasoning. If both are not... then "the ball's in your court."
But "good idea?" No, either way. But you just have to measure age v. eternity. There are quite a few 'ideas' that aren't good for one or the other-- starting with Jesus dying on the cross.
That in bold has caused me to give some thought concerning the righteousness of God.
One would not be judged for another. Or the son shall not die for the sin of the father or the father for the sin of the son. For all will die in their own sin.
However God brings forth a man child Son born of woman. That Son is sinless and so being never the less is obedient unto death, even the death on the cross. Therefore God, his Father gives this sinless Son who gave his life for others the promise of God. The hope of eternal life.
The resurrection to life to die no more becomes the righteousness of God for the faith/faithfulness of Jesus the Christ his Son. Romans 6:9 Knowing that Christ being raised from the dead dieth no more; death hath no more dominion over him.
And he went a little further, and fell on his face, and prayed, saying, O my Father, if it be possible, let this cup pass from me: nevertheless not as I will, but as thou wilt. Matt 25:29
Saying, Father, if thou be willing, remove this cup from me: nevertheless not my will, but thine, be done. Luke 22:42
The sinless Son of God is the only one who could redeem but that redemption required obedience unto death even the death of the cross. And scripture states Jesus the Christ learned obedience and became obedient.