Could you please find scriptures in the Bible that when a parent or step-parent mistreats infants and young children, what will their consequences be?
The Bible condemns those who mistreat any other person, whether child or adult. The Bible even teaches us (Deuteronomy 25:4; Deuteronomy 22:6-7) not to mistreat animals.
The responsibility of parents not to mistreat their children is specifically mentioned in Ephesians 6:4. "And, ye fathers, provoke not your children to wrath: but bring them up in the nurture and admonition of the Lord."
I think of two passages that may deal directly with the consequences of mistreating others, including children.
"And even as they did not like to retain God in their knowledge, God gave them over to a reprobate mind, to do those things which are not convenient; Being filled with all unrighteousness, fornication, wickedness, covetousness, maliciousness; full of envy, murder, debate, deceit, malignity; whisperers, Backbiters, haters of God, despiteful, proud, boasters, inventors of evil things, disobedient to parents, Without understanding, covenantbreakers, without natural affection, implacable, unmerciful: Who knowing the judgment of God, that they which commit such things are worthy of death, not only do the same, but have pleasure in them that do them." (Romans 1:28-32)
Paul says anyone who is without natural affection, who is unmerciful, who is malicious (to mention the items specifically related to mistreating children or others) is worthy of death.
"Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness, Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies, Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God. But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, Meekness, temperance: against such there is no law. And they that are Christ's have crucified the flesh with the affections and lusts. If we live in the Spirit, let us also walk in the Spirit. Let us not be desirous of vain glory, provoking one another, envying one another." (Galatians 5:19-26)
Among those who will not inherit the kingdom of God are those who hate, who create strife, and who are angry. In contrast, followers of God will be loving, peaceable, gentle, and have their passions under control.