[{"_id":1,"name":"name1","quizzes":[{"_id":1,"name":"q1","questions":[{"title":"t1"},{"title":"t2"}]},{"_id":2,"name":"q2","questions":[{"title":"t3"},{"title":"t4"}]}]}]

db.collection.update({"_id":1,"quizzes._id":1},{"$push":{"quizzes.$.questions":{"title":"newTitle"}}})