Will I be credited for forgiving someone who wronged me in my childhood, merely because I felt sad for him in a certain situation, even though I did not forgive him verbally? And will I be sinful if I supplicate against him now?

It is permissible to supplicate against an oppressor to the extent of their injustice, but it is not permissible to transgress in the supplication beyond that. Forgiveness and forbearance are better and yield a greater reward; for whoever forgives and reconciles, their reward is with Allah, and it is closer to piety. It increases a servant in honor and is a reason for Allah's forgiveness of the servant, as stated in His Almighty قول (statement): "And let them pardon and overlook. Would you not like that Allah should forgive you? And Allah is Forgiving and Merciful."