Where does Mark's Gospel begin? With the baptism of Jesus—that is, with his public life and ministry. The fact that Mark does not write about the birth and childhood of Jesus in no way invalidates the facts related by Matthew and Luke. It discredits them no more than McMaster, in his history of the United States which commences with the year 1784, invalidates the facts about the Colonial history of the United States which are related by Bancroft.
You might as well argue that there was no Declaration of Independence and no Bunker Hill because there is no mention of these events in a history of the United States which commenced with the Civil War as to argue that there was no virgin birth because Mark, who records the public life of Jesus, makes no reference to it. The birth and childhood of Jesus lay outside the scope and plan of his treatise.