I would contact a lawyer, and you'll need a paternity test, etc. to determine if he is the father.
That being said, I don't think your marriage has much to do with this child. He may need to provide support, but that money will be taken away from your children whether you are married or divorced. It will probably take less away from them if you stay married.
Having unprotected sex is so common in affairs - it is shocking, so don't feel like you are alone in this. You are right, this little girl is an innocent, so she needs to be provided for regardless of the situation she was born into. SO sorry about what you are going through, but it is just by the sheer luck, and possibly aging sperm and eggs in our case, that I (and many of us) are not in your shoes.