Actually Jim, the description of the experiment I saw in an old science text book was pretty much as you describe which is why I designed the plexiglass box. As I said it was approx. 1 metre cubed, and sealed with silicone except for the input and outlet holes ~1" D. And yes, you are exactly right, there are [invisible] currents in the CO2 "atmosphere" just like there are currents in the "real" earths atmosphere. Many large thunderhead clouds have been reported by airline pilots to have updrafts many thousands of feet higher than the regular airline altitudes.
You should really read up on the physics of sunlight and how it heats the earth, especially if you are home schooling your daughter. What happens in the most simplistic terms is the sunlight hits the darker portions of the earth's surface, heats up those portions of the earth's surface, then that heat radiates upward into the atmosphere, and is both radiated out of the atmosphere and convected upward by air currents where it then radiates out of the atmosphere.
If you wanted to be really accurate, you could mount a black ball in the plexiglass box and shine a heat lamp from outside the box on it. You'll get the same results, that is if you can keep the heat lamp's light off the thermostat's pickup by using the aluminum pie plate.
That's one of the major things that is happening because of climate change. It's called a "runaway" reaction. As the ice and snow (ice and snow are white and reflect most of the sun's radiation) melts earlier in the year, there is more dark surface to absorb heat, this melts more snow and ice, resulting in yet more dark surface resulting in more heat and so on and so on. When enough ice and snow are gone, we reach what is called the "tipping point" and then the hockey stick graph of heating really turns up. Have your daughter read "Gaia's Revenge" (or maybe it's the revenge of Gaia, I forget the exact title) by Lovelock, it's a good, interesting read, and it'll encourage alternate thinking on her part.
And, Yes, I encouraged alternate thinking in the classroom. That's science! however, facts are facts and my students mostly understood that opinions don't trump facts, not in a Science classroom.
I've been following the Copenhagen summit and am really disgusted with it. No one is mentioning the many other parts of the whole system that are running amok. The world's deforestation, the increase in the number of automobiles, the decrease in the oceanic coral reefs, the increasing extinctions both in the ocean and on land, the horrific problem with fresh water, etc. etc. etc. All these and more phenomenon are showing textbook "hockey stick" graphs and are all related. Just taking action to mitigate co2 release by petroleum products isn't going to be nearly enough.
And no one is even whispering about the "Elephant in the Room"!
Stan