niteowl, let's face it, you daughter is not going to stop wanting to trace her mother, is she? Not ever? And when she does, what is she to think of you for standing in her way, the more so if, when she does, she finds that her mother has died, she can never meet her mother again and has only whatever memories she has as at 5 years old. That would be a terrible thing to live with.
What reason to you have for denying her? Was her mother a murderess? Drug addict? Or did she simply leave you, say, for someone else? Or is simply that you are still angry at her mother ? Whatever the reason, the woman is still her mother ! As Frank says, your daughter will always know and appreciate all you have done for her. You may not think that list of good things will be improved by addding 'My father always stopped me meeting my own mother!"