You may change your Captain from quiz to quiz. But once a captain is chosen for that quiz, you must stay with that captain for that quiz. If the captain leaves the platform during a quiz, the Co-captain should be identified for the officials.
Please remember there are many factors to consider when choosing a captain – leadership skills, enthusiasm, encouragement, maturity, etc. But during the actual tournament the officials need to know who your captain is since that person is the only one who can challenge a question. If another member of the team “urges” the captain to challenge a question, that challenge will be denied since it did not initiate from the captain. And please, Coaches, there is to be no prompting involved. If a coach does prompt, the challenge will also be denied according to the same rule.
If a captain wants to challenge a question, he or she should stand, and declare that they want to challenge the question and on what basis they are challenging. They should not just stand and ask a question about what was said. If they do the Quizmaster will ask them if they are challenging the question and on what basis. 
Also, when that Official has given his or her ruling, there should be no arguing with them at that point. They are the authority in the room (no matter what their age). It can happen at tournaments and at international competitions when quizzers or coaches begin to argue a ruling by an official. I do not believe it is a good example for us to be setting for our kids. One of the lessons learned in quizzing is to respect authority and it’s great when coaches lead in this area. If you have a concern with a particular official or a particular ruling, please bring it to the CBQ Board (Michael, Sharlene, Hunter, or Jane) and please do it out of earshot of others. I realize we can get compassionate about how our kids are doing at the competition, but please remember that the day is most likely not going to go perfectly. You always have the option to put a quiz under protest. Just remember that all officials are volunteering their time and money just as you are, so please treat them with the respect they deserve. Let us be Christ-like in our behaviour even in the heat of the competition.

A special thank you to Kathy for allowing us to use her words to assist our Captains and Coaches when it comes to Challenging, and dealing with authorities within Quizzing.