You have come painfully close to the evidence that we live in a fallen world. Evil done to children I think is the hardest for anyone with a heart to bear. You will need to give this into the Lord’s hands and trust Him to comfort & keep you.
This is however, no reason to throw in the towel. The Lord tells us to pray for God’s will on earth because it is one way we do battle whit what 2Corinthians 4:3-4 refers to as “the god of this world” (But if our gospel be hid, it is hid to them that are lost: In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious gospel of Christ, who is the image of God, should shine unto them.)
These are the very things that break our heavenly Fathers heart. You are on the front lines and so you will suffer & bear this pain with our Lord. This is an honor (to share His heart & suffer with Him) even though it feels unbearable at times.
I think that there are certain rules in the spiritual realm that God has not fully disclosed to us. I think we will understand in the after life how these “rules” allowed the powers of darkness to do evil and how we can at times counter this with prayers and fasting.
Satan would love for these things to discourage us to the point of surrender, but this we must not do. We will have all the answers we seek in the end but for now Trust God,and go to the tree of life, and not to the tree of knowledge demanding the answers to WHY.
The general answer is that we live in fallen world where many are under the influence and or captives of darkness and many innocents suffer because of this fact. God is in control of all who are submitted to Him, but unfortunately that is not all of us. This is a war, retreating is not an option.