It doesn't look like people sitting in the 3rd row have much of a view with the windows. I think my kids might get car sick without a true window. So I think the back roof needs to be squared more which will also give space for tall boxes.
They'd have a great view of the sky, though.
Maybe they will come up with a 4th row rear facing bucket seats like in the MS. Problem solved. Rearview window is huge.
That would mean the car is maxed out with 9 passengers.
If you have ever been car sick, you'll know that you can't face the rear in a moving vehicle.
The reason why the third row side windows are so small is that the rear doors are very large. They allow for easier entry and exit to the third row by having a big opening. If those doors were smaller, the rear pillar to support them would be further forward, allowing a larger window. The trade-off would be that you no longer would have the easy access to the last row, even with the Falcon Wing Doors.
The alternative would be to make the Model X even longer, to make those side windows larger. That would effectively make the vehicle about the length of a full SUV, rather than the crossover it is meant to be. At that size, you would have more weight. More weight means more energy being used per charge. That would lead to shorter range overall.
Unless or until Tesla Motors is able to develop a significantly higher capacity of battery pack, I doubt they will make anything that large. The Tesla Model S and Model X are each rather heavy as it is, due to the weight of the batteries. The third row on the Model X is large enough to hold adults. So your kids can ride in the second row just fine.
I am comparing the Model X with minivans, but I like your explanation, Red Sage. Being a soccer mom, I do drive lots of kids at one time so the ease of entry is important too.