"This is your child."
Translation:Это твой ребёнок.
Biological gender doesn't indicate grammatical gender.
Consider these examples:
In French, both "child" (enfant) and "baby" (bébé) are masculine nouns, regardless of the biological gender of the child/baby.
In German, the grammatical gender of "cat" (Katze) is feminine regardless of any particular cat's biological gender, whereas the noun, "girl," (Mädchen) is a neuter noun.
And, in Norwegian, any feminine noun can be declined in both feminine and masculine forms, including nouns such as "woman" (kvinne) and "girl" (jente).
You look at the noun. For example "Your book" becomes "Твоя книга". While "Your books" becomes "Твои книги". If the noun (in this case book - книга) is in singular you use either твой, твоя, твоё to say your depending on what gender the noun is. If the noun is in plural you use твои to say your and also does not depend on the gender of the noun.