How do you reconcile that the Bible explains many times in the Old Testament that people should not pass their children through the fire.
One basic, I would say, is God wanted humans to reproduce and replenish the earth . . . right? So, you can see that killing children would not be doing that. That would be against God's purpose.
But Jesus on the cross was acting for God's purposes. He even said it would be better if He left this earth . . . better for His disciples.
"'Nevertheless I tell you the truth. It is to your advantage that I go away; for if I do not go away, the Helper will not come to you; but if I depart, I will send Him to you.'" (John 16:7)
And likewise god did not want us to sacrifice our children even to him.
God does not want us to only make big gestures that seem impressive to us!
He desires that we become sacrificed to loving and forgiving and being gentle and humble. Sacrifice that self-ego! And become alive in Jesus' own love living in us. This comes with the Holy Spirit in us.
"Now hope does not disappoint, because the love of God has been poured out in our hearts by the Holy Spirit who was given to us." (Romans 5:5)
God gives his only begotten son to atone for our sins. Why would God forbid his own people to do such a thing?And then he would do it himself?
Partly, because humans can't do what God was doing, by sacrificing their children. Instead, God desires that humans be good examples to help their children to grow up Christlike . . . gentle and humble and all-loving, sharing and caring as family, and generously forgiving. Just killing them would be "lazy" . . . not taking on the real challenge and education and sacrifice which our Father desires to share with us > how, with God, we are able to find out how to love and how to bring up children to know how to love.
In the case of Jesus dying on the cross, God had the purpose of saving us from our sins.
Also, Jesus was being our example of how to love, by how He was loving on the cross > we are commanded to follow His example >
"And walk in love, as Christ also has loved us and given Himself for us, an offering and a sacrifice to God for a sweet-smelling aroma." (Ephesians 5:2)
We, with Jesus, need to honor and obey His example, by ourselves being His example of how to love, so we and our children can feed on our example of how to relate in God's love > including >
"without complaining and disputing" (in Philippians 2:14) . . .
"with all lowliness and gentleness, with longsuffering" (in Ephesians 4:2) . . .
"forgiving one another, even as God in Christ forgave you" (in Ephesians 4:32).
Jesus went through things of this life so now He can feel for us and minister to us His grace which made Him able to do so well >
"For we do not have a High Priest who cannot sympathize with our weaknesses, but was in all points tempted as we are, yet without sin." (Hebrews 4:15)
So, included in going through the cross is how Jesus was getting experience with problems of this life, so now He can feel for us while we go through things for Him.
And He is our example, too, in this, of how we can with God make good use of things we go through, using our troubles and suffering to help us to feel for others and minister to them how God's grace makes us able to do well.
Plus, in the process of dying, Jesus defeated death and by means of His death will destroy Satan >
"Inasmuch then as the children have partaken of flesh and blood, He Himself likewise shared in the same, that through death He might destroy him who had the power of death, that is, the devil, and release those who through fear of death were all their lifetime subject to bondage." (Hebrews 2:14-15)
And there is more, I believe, which you can discover by sharing with Christians and exploring God's word prayerfully.
It is interesting how a human can do less, by dying, but Jesus has done more, by dying. So, thank You, Jesus, for all You have done!!!