This is one of the hardest questions that adoptive parents ask me. It is layers deep and generations long. It goes into the big issues of acceptance and love, guilt and anger, and the ever-present fear of being wrong that plagues so many people. The best answer I have for these parents is to ask more questions and lead them into their own answers. The main thing I help them question is their motivations. "Is God building your family or are you trying to do it in spite of Him and in your own strength and wisdom?" Outside of God, I think there are mistakes made in adoption plans. With him, I trust there are not.
The same basic question can be changed to cover many different adoption situations -- it can apply to children who are older, disabled, in foster care, abused or neglected. If our answer to the question is "Yes, we are following God's lead and planning in this," then we can rest in Him because he makes no mistakes.