Robert,
I have 4 kids 2 in car seats. I have both the 4Runner with 3rd row and the GX with third row. The GX470 is the same vehicle you are presently driving, except it has the 3rd row option. They surprisingly cost less than a 4th gen of the same vintage and you get to keep the V8. My friends GX460 has tumble 3rd row and IMHO the drop down version in my GX are far better.
If I can be so bold as to point something out.
In the event of a side impact, your side curtain air bags and seat air bags keep the occupants from jolting to one side or the other. By sitting your child in the middle, you may save them from an event where there are crushed by a hit hard enough to for the impacting vehicle to enter the cabin (I have never seen a side impact on a 120/150 platform where another motor vehicle actually penatrated into the cabin). However, what you have done is ensured that no mater the severity of the side impact, their little body will have to take the full force af the lateral movement caused by the impact. Simply put, the airbag will be to far away to cushen them and they just bounc around.
Worst case scenario with the car seats on the side is a direct hit on that exact spot causing blunt force trauma. However, with the car seat in the middle....every accident is the worse case scenerio, and the risk of internal decapitation may be increased.
FWIW, my wife works for State Farm (contract) and I am a former Police Officer. I have seen my share of accidents up close, there is no sure bet.