After reading your post, it got me to thinking (which can sometimes be dangerous). If you have a child in the back and have the childproof locks engaged, that is a good thing. However, if you have them engaged when adults are in the back, it could lead to unintended consequences. If, God forbid, you were to have an accident where the driver is incapacitated, the rear seat passengers could then be trapped in the car. It may be a pita to engage/disengage the child proof locks as needed, but it is something to think about.
It would be more convenient if the locks could be engaged based on passenger weight, wherein if a passenger weighing more than, say, 85lbs would disengage the locks, or at least be able to do so from the driver's seat.