Thank you for bringing your very difficult and sensitive question to this forum. I am hopeful that you and others will be able to gain a great deal from reading and responding to your post. You are raising a very important topic, one which warrants more input and feedback than I can provide in this short format, but I hope that I can give you some issues to consider.
It sounds like you are leaning toward telling your daughter that her father is not her biological father. I think that this is a very courageous decision because secrets can be very deleterious in families. In families with secrets, there is generally less open and honest communication. Open communication fosters closeness and intimacy, so keeping secrets often sacrifices bonds and relationships.
Also, in my own experience with therapy clients, secrets have a way of expressing themselves, although usually in negative ways. Children have a way of "knowing" even when they don't really "know." What do I mean by that? I am suggesting that children can tell when things do not "add up." They can tell when the mood in the room changes because a topic related to the secret has emerged. They understand that the adults in the room are tense, but they are not sure why. In a young child's mind this can lead to anxiety and uncertainty. The child consciously or unconsciously thinks, "The adults seem upset because of something to do with me. I must be bad. There must be something wrong with me." These thoughts are manifested in fears and/or acting out behavior.
That said, I also think that there is a timing and process for communicating family secrets. I wonder whether your daughter is at the right age to share this kind of information with her. She may be a bright girl, but I'm not certain that she will be able to fully understand the issues. Cognitively, children at age 6 are typically more egocentric, meaning they have difficulty distinguishing their point of view from the point of view from others. It will be difficult for her to imagine mommy and daddy's motivations and feelings. Also, children at age 6 are prone to magical thinking. Thus, I wonder about the timing of telling your daughter now. She may do better to learn about this secret when she is older, perhaps nine or ten years old?
I do think that it would be useful to consult with a mental health professional as you proceed to share the secret with your daughter. It might be ideal to tell your daughter within the context of a therapy session and/or your daughter may benefit from meeting with a mental health professional afterward for a while to process the information.
I have included a link below from an online discussion that is very similar to your situation. The second link is for a book that explores family secrets. I have not read the book, but I think that it may be worthwhile for you to explore.
Good luck and please let us know what you decide. You are in my thoughts!
L. Compian, Ph.D.
Education.com Expert Panel
Be honest . I am an adopted child and I found out I was adopted through someone other than my parents and it hurt me to the point of know return. Honesty I think always works because you really can't go wrong with the truth.
Apologize.For not telling the child earlier and for your mistakes.
Reasure. That they were not a mistake and he/ she waass all apart of gods devine plan.
Remind. the child how he /she is loved and how they have just gained more family and friends.