I'm sorry, what? I've never heard of such a feature. My googling only lead to "airbags are a danger to children" links, I'm afraid the bolded section I've doubts on. Do you have a link to this feature?
If they don't have such an ability yet, than it really would be a new tech they'd have to develop, and really, statistically does it make sound sense to do so? How many people would actually use the feature, and also be the type of person who would forget about their child in the backseat? If they're a forgetful person, probably they'd also forget to turn the feature on.
Above argument applies. How many people would have this feature, remember to turn this feature on, and also forget their child?
I will say that this one of sensing the doors opening/closing would make more sense than a weight limitation alone. It'd elliminate the all/nothing weight allotment, and car would only have to install a simple weight scale and some software to enact, vice build a whole new system to calculate when a child is or is not in the safety seat that is constantly left in the car.