dward1 -
What Frank said. You can cover the upper grill and be "aggressive" with the lower grill too. I am thinking that you could probably get away with just the center two slats open for winter :
Try tape at first and monitor coolant temps. If it's too hot, peel off successive layers of (vertically placed) tape, and repeat the test until you reach temps that are comfortable for you.
Does your Sienna have a roof rack? It looks like a stock photo, is why I asked. Would removing that be too much for the parental units?
CarloSW2