If you are the child's natural father and you marry her mother within the child's lifetime, and you are already on her birth certificate as the father, then you can ask the registrar to re register the birth as a legitimate birth in retrospect after you are married. It is confusing but the registrar will explain the necessary application. In the mean time, add your surname to the end of her current name by using a hyphen, it will be legal if both your's and your partners names appear on her birth certificate. It is better to start to do this now to establish use. For a passport however it would not be valid.