No Positive Outcome
It’s possible that the child would never discover those parents, which could cause a lot of unpleasant feelings like sadness and even depression. The same holds true if she were to run into those parents and they rejected her.
Not A Good Idea
Alternately, it might go the opposite way. Perhaps the youngster locates her parents, and they will begin to reestablish their relationship. Their personal relationship with their adopted daughter will suffer as a result. No matter how you look at it, allowing the girl to look for her parents was a mistake.