Can one repent of the crime of child sexual abuse committed in one of God’s houses? And is it necessary to seek forgiveness from the child and their parents? And does this crime fall under “striving for corruption”? And how can one expiate for it and attract God’s mercy?

What you did is very ugly, but repentance erases the sins that preceded it. If your repentance is sincere, then you are like one who has not sinned, for the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin." So turn to Allah with sincerity and truthfulness, regret what you have done, increase in good deeds that wipe away sins, adhere to seeking forgiveness, and have a good opinion of your Lord, the Most High, for He is Forgiving and Merciful. Do not profane the child, and do not tell his father, but rather conceal yourself with Allah's concealment, improve what is between you and Allah, and strive in seeking forgiveness and doing good deeds.