What is the ruling on placing a part of the Noble Qur’an in the private parts while committing an immoral act in childhood, and does the ruling differ if the perpetrator was an adult or not, and what should the perpetrator do after remembering this act after becoming committed to religious practice?

What the questioner mentioned is strictly forbidden. Jurists have stated that whoever commits such an act apostatizes. Thus, anyone who shows contempt for the Quran or the Mushaf (physical copy of the Quran), or any part of it, or attempts to dishonor it, such as throwing it into filth, has committed an act of disbelief.

Allah's mercy is vast. If this act occurred after puberty, then she must offer sincere repentance, for repentance erases the effect of sin, and one who repents from sin is like one who never sinned. Allah Almighty said: "And indeed, I am the All-Forgiving to those who repent, believe, and do righteous deeds, then are guided" (Taha: 82). And He Almighty said: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" (Az-Zumar: 53).

She must conceal this and not inform anyone about what she did. Whoever commits such defiling acts should conceal themselves with Allah's concealment. However, if the act occurred before puberty, it is not recorded against her; for the pen is lifted from the child until they reach puberty.