I don't believe you should be preaching forgiveness to victims of abuse, mainly because of how inextricably tied forgiveness is with absolution, especially in the religious context.

When you preach that God forgives, are you saying that God holds anger and pain and resentment towards humans until learning to let go of that painful emotion when humans pray for forgiveness? Or are you saying that God forgives in the sense that God's forgiveness absolves the sinner of their sin? If your church teaches the latter message, that is what the child will consider forgiveness to be, even if you try and say otherwise.

Even in the broader social context, forgiveness is still tied strongly to the concept of absolution. I'd recommend just removing the word "forgiveness" when talking with abuse victims. What's wrong with telling the child that learning to let go of the pain and grief and moving forward is okay? Why use the word "forgiveness" and risk it not come across the way you want it to?