I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today.
You asked: Whats my best defense to win the case
I hope that I don't need to tell you just how dangerous it is to seat belt a child or infant into the lap of another passenger. Children are much more easily seriously injured and even killed in a collision when seat-belted in this manner.
Unfortunately, you don't have a defense to the charge. CA seal belt law requires that each occupant have their own seat and their own seat-belt. Tandem seat belting is not allowed and is still in violation of the law.
If you can pay the fine without appearing in court, then just pay it and be done with it. If you have a mandatory appearance, then just plead guilty and explain to the judge that you won't ever do that again, and that you have learned that tandem belting is very dangerous and you were not aware of that before you were stopped.
You may reply back to me using the Reply link and I will be happy to continue to assist you until I am able to address your concerns, to your satisfaction.
I hope that I have been able to fully answer your question. Please be so kind as to rate my service to you. That is the only way I am credited for assisting you.
I wish you and yours the best in 2016,