¿A dónde _tu hijo con mi hija?
One choice was va and one choice was vas. Why va and not vas?
"va" is 3rd person (he), while "vas" is 2nd person (you). The question is "Where does your son go with my daughter?". The subject is "tu hijo" (your son = he) so it is 3rd person.
Because the subject of the verb is "tu hijo" which is in the third person and takes the conjugation form of va. If the subject of the verb in the sentence was "tú", then vas would be correct.